Test for PyData.ClassName currently fails for py3_pickle3 zkind:

    === RUN   TestPyClassName/py3_pickle3
        pydata_test.go:50: class name for "\x80\x03X\x18\x00\x00\x00ZODB.tests.testSerializeq\x00X\x13\x00\x00\x00ClassWithoutNewargsq\x01\x86q\x02N\x86q\x03.":
            have: "?.?"
            want: "ZODB.tests.testSerialize.ClassWithoutNewargs"
        pydata_test.go:50: class name for "\x80\x03X\x18\x00\x00\x00ZODB.tests.testSerializeq\x00X\x10\x00\x00\x00ClassWithNewargsq\x01\x86q\x02K\x01\x85q\x03\x86q\x04.":
            have: "?.?"
            want: "ZODB.tests.testSerialize.ClassWithNewargs"

This happens because classname decoder was expecting class and module
names to come as py2 bytestrings, while py3 emits them as unicode.

-> Fix it by accepting both py2 bytestring and unicode for class and
module names.

Previously we were testing PyData only with data generated by python2
and pickle protocol=2. However even on py2 there are more pickle
protocols that are in use, and also there is python3.

-> Modernize py/pydata-gen-testdata to use run_with_all_zodb_pickle_kinds
   that was recently added as part of nexedi/zodbtools@f9d36ba7
   and generate test data with both python2 and python3. It is handy to
   use py2py3-venv(*) to prepare python environment to do that.

   Adjust tests on Go side to verify how PyData handles all generated
   zkinds.

py2_pickle1, py2_pickle2 and py2_pickle3 are handled well.
Tests for py3_pickle3 currently fail and so are marked with "xfail".

We will fix tests for py3_pickle3 in the next patch.

(*) see nexedi/zodbtools@fac2f190

Tests, that want to verify something wrt both py2 and py3 will do
WithEachPy(tested_func) which will run tested_func two times:

1) under environment where python on $PATH points to python2, and
2) under environment where python on $PATH points to python3.

Client tests will now need to use just "python" instead of e.g.
"python2" or "python3" to run some python program, and each time
"python" will correspond to current phase of WithEachPy execution.

This will be soon handy to test things wrt e.g. ZEO/py2 and ZEO/py3
servers and similar situations.

Tests that merely want to use some python program just for their inner
working, for example to run `zodb commit`, no longer indicate their
preference for py2. For such tests it is a matter of preference in
pre-setup environment to where "python" points.

For the reference under environments created with py2py3-venv(*) default
"python" currently points to "python2".

(*) see nexedi/zodbtools@fac2f190

ZTestData is Go counterpart of ztestdata on zodbtools/py side from
https://lab.nexedi.com/nexedi/zodbtools/-/blob/f9d36ba7/zodbtools/test/conftest.py#L31-65
that was recently added in nexedi/zodbtools@bf772ce0.

On Go side each ZTestData describes testdata files on the filesystem +
carry arbitrary extra information. For example FileStorage tests will
use it to place there "what is expected to load" information
corresponding to on-filesystem database.

Currently with the only utility to retrieve map keys in sorted order.

We will need this in several tests in follow-up patches.

Similarly to ZODB/py which provides u64 and p64 utilities. Add
zodbpickle.Pack64 for symmetry.

Because without StrictUnicode both bytestrings (str from py2) and
unicode (unicode from py2 and str from py3) load into the same Go type
string so it becomes impossible to distinguish them from each other.
Re-saving data, thus, also generally introduces changes as e.g.
string loaded via bytestring will be saved as unicode when pickling with
protocol 3. Or a loaded unicode will be saved as bytestring with
pickling via protocol=2.

-> Switching to StrictUnicode mode solves all those problems.

Please see updated documentatin for zodbpickle.go and
https://github.com/kisielk/og-rek/commit/b28613c2 for more details about
StrictUnicode mode.

For ZODB/go this is change in behaviour exposed to outside. However
there is currently only one known ZODB/go user - WCFS in Wendelin.core -
and that user will be updated correspondingly as well.

We already have 3 places where we create picklers and unpicklers:
zodb/pydata.go, fs1/index.go and zeo/proto.go and they are already
diverging a bit: for example pydata was explicitly specifying protocol
for pickle encoder, while other places were using defaults.

We will soon want to use StrictUnicode option for all picklers and
unpicklers we create, which means it is better to keep this
customization only in one place instead of copy-pasting it in between
callsites increasing the possibility for divergence and errors.

-> Move the code to create pickler and unpickler to internal zodbpickle
package as a preparatory step for that.

For the reference: this package is merely a wrapper around ogórek, not a
fork of any code like zodbpickle/py is.

We will soon want to use generics and `any` which are available only for go >= 1.18 .

The other changes in go.sum and go.mod are there because after just

    -go 1.14
    +go 1.18

the build stopped to work and asked to do `go mod tidy`

ogórek.AsInt64 was added in https://github.com/kisielk/og-rek/commit/010fbd2e
with functionality similar to what we had in pickletools.Xint64 before.

This brings in support for StrictUnicode mode, AsInt64, AsBytes and
AsString helpers, and other fixes.

See https://github.com/kisielk/og-rek/compare/v1.2.0...7d520718eea5 for
details.

So that building and using ZODB/go works not only in GOPATH mode.

Offload drivers from handling options such as ?read-only=1 and force
them to deal with such options only via DriverOptions, never zurl.

See added comment for details.

/reviewed-by @levin.zimmermann
/reviewed-on !4

go/neo/neonet: Fix client handshake not to accept server encoding if it is different from what client indicated

If the peers encoding is different than our encoding two different
scenarios can happen, because the handshake order is undefined (e.g.
we don't know if our handshake is received before the peer sends its
handshake):

1. Our handshake is received before peer sends its handshake, NEO/py
closes connection if it sees unexpected magic, version, etc.

2. The client already sends a handshake before it proceeds our handshake.
In this case it initally sends us it version, we can extract its encoding,
and only later, once it proceeded our handshake with the bad encoding,
it closes the connection.

Before this patch case (2) wasn't handled correctly by the automatic
encoding detection of 'DialLink'. 'DialLink' simply accepted the
different-than-expected encoding, but once the peer proceeded the nodes
handshake the peer closed the connection and the initially established
and returned link was immediately closed again. Due to this it was good
luck whether connecting with a peer different with an encoding different
from the expected one worked or didn't work (it depended on which handshake
was faster). Now 'DialLink' should reliably find the correct encoding
and return a stable link.

--------

kirr: this is based on the following original patch by Levin: levin.zimmermann/neoppod@f6b59772

I updated documentation throughout correspondingly and also added
corresponding handshake-specific test in the previous patch.

See !5 and b2da69e2
(go/neo/neonet: Demonstrate problem in handshake with NEO/py) for more
in-depth description of the problem.

Levin Zimmerman discovered that sometimes NEO/py accepts our handshake
hello with encoding 'M', then replies its owh handshake ehlo with
encoding 'N' and then further terminates the connection. In other words
it looks like that the handshake went successful, but it actually did
not and NEO/py terminates the link after some time.

This manifests itself e.g. as infrequent TestLoad failures on t branch
with the following output:

=== RUN   TestLoad/py/!ssl
I: runneo.py: /tmp/neo776618506/1 !ssl: started master(s): 127.0.0.1:21151
=== RUN   TestLoad/py/!ssl/enc=N(dialTryOrder=N,M)
    client_test.go:598: skip: does not excercise client redial
=== RUN   TestLoad/py/!ssl/enc=N(dialTryOrder=M,N)
    xtesting.go:330: load 0285cbac258bf266:0000000000000000: returned err unexpected:
        have: neo://127.0.0.1:21151/1: load 0285cbac258bf266:0000000000000000: dial S1: dial 127.0.0.1:40345 (STORAGE): 127.0.0.1:56678 - 127.0.0.1:40345: request identification: 127.0.0.1:56678 - 127.0.0.1:40345 .1: recv: EOF
        want: nil
    xtesting.go:330: load 0285cbac258bf266:0000000000000000: returned tid unexpected: 0000000000000000  ; want: 0285cbac258bf266
    xtesting.go:330: load 0285cbac258bf266:0000000000000000: returned buf = nil
    xtesting.go:339: load 0285cbac258bf265:0000000000000000: returned err unexpected:
        have: neo://127.0.0.1:21151/1: load 0285cbac258bf265:0000000000000000: dial S1: dial 127.0.0.1:40345 (STORAGE): 127.0.0.1:56688 - 127.0.0.1:40345: request identification: 127.0.0.1:56688 - 127.0.0.1:40345 .1: recv: EOF
        want: neo://127.0.0.1:21151/1: load 0285cbac258bf265:0000000000000000: 0000000000000000: object was not yet created
    ...
    client_test.go:588: NEO log tail:

        log file 'storage_0.log' tail:

        2023-07-17 17:21:57.1519 DEBUG     S1         connection completed for <ServerConnection(nid=None, address=127.0.0.1:51230, handler=IdentificationHandler, fd=20, server) at 7f3583fd4f50> (from 127.0.0.1:40345)
        2023-07-17 17:21:57.1537 WARNING   S1         Protocol version mismatch with <ServerConnection(nid=None, address=127.0.0.1:51230, handler=IdentificationHandler, fd=20, server) at 7f3583fd4f50>
        2023-07-17 17:21:57.1548 DEBUG     S1         connection closed for <ServerConnection(nid=None, address=127.0.0.1:51230, handler=IdentificationHandler, closed, server) at 7f3583fd4f50>
        2023-07-17 17:21:57.1551 WARNING   S1         A connection was lost during identification
        2023-07-17 17:22:00.1582 DEBUG     S1         accepted a connection from 127.0.0.1:51236
        2023-07-17 17:22:00.1585 DEBUG     S1         connection completed for <ServerConnection(nid=None, address=127.0.0.1:51236, handler=IdentificationHandler, fd=20, server) at 7f3583fd4e90> (from 127.0.0.1:40345)
        2023-07-17 17:22:00.1604 WARNING   S1         Protocol version mismatch with <ServerConnection(nid=None, address=127.0.0.1:51236, handler=IdentificationHandler, fd=20, server) at 7f3583fd4e90>
        2023-07-17 17:22:00.1622 DEBUG     S1         connection closed for <ServerConnection(nid=None, address=127.0.0.1:51236, handler=IdentificationHandler, closed, server) at 7f3583fd4e90>
        2023-07-17 17:22:00.1625 WARNING   S1         A connection was lost during identification
        2023-07-17 17:22:03.1663 DEBUG     S1         accepted a connection from 127.0.0.1:51238
        2023-07-17 17:22:03.1666 DEBUG     S1         connection completed for <ServerConnection(nid=None, address=127.0.0.1:51238, handler=IdentificationHandler, fd=20, server) at 7f3583fd4d10> (from 127.0.0.1:40345)
        2023-07-17 17:22:03.1674 WARNING   S1         Protocol version mismatch with <ServerConnection(nid=None, address=127.0.0.1:51238, handler=IdentificationHandler, fd=20, server) at 7f3583fd4d10>
        2023-07-17 17:22:03.1688 DEBUG     S1         connection closed for <ServerConnection(nid=None, address=127.0.0.1:51238, handler=IdentificationHandler, closed, server) at 7f3583fd4d10>
        2023-07-17 17:22:03.1691 WARNING   S1         A connection was lost during identification
        2023-07-17 17:22:06.1714 DEBUG     S1         accepted a connection from 127.0.0.1:57072
        2023-07-17 17:22:06.1719 DEBUG     S1         connection completed for <ServerConnection(nid=None, address=127.0.0.1:57072, handler=IdentificationHandler, fd=20, server) at 7f3583fd4b50> (from 127.0.0.1:40345)
        2023-07-17 17:22:06.1727 WARNING   S1         Protocol version mismatch with <ServerConnection(nid=None, address=127.0.0.1:57072, handler=IdentificationHandler, fd=20, server) at 7f3583fd4b50>
        2023-07-17 17:22:06.1738 DEBUG     S1         connection closed for <ServerConnection(nid=None, address=127.0.0.1:57072, handler=IdentificationHandler, closed, server) at 7f3583fd4b50>
        2023-07-17 17:22:06.1738 WARNING   S1         A connection was lost during identification

        log file 'master_0.log' tail:

        2023-07-17 17:21:21.0799 PACKET    M1         #0x0012 NotifyNodeInformation          > A1 (127.0.0.1:37906)
        2023-07-17 17:21:21.0799 PACKET    M1          ! C0 | CLIENT |  | RUNNING | 2023-07-17 14:21:21.079838
        2023-07-17 17:21:21.0800 PACKET    M1         #0x0102 NotifyNodeInformation          > S1 (127.0.0.1:37918)
        2023-07-17 17:21:21.0800 PACKET    M1          ! C0 | CLIENT |  | RUNNING | 2023-07-17 14:21:21.079838
        2023-07-17 17:21:21.0801 DEBUG     M1         Handler changed on <ServerConnection(nid=None, address=127.0.0.1:37966, handler=ClientServiceHandler, fd=18, server) at 7f3584245910>
        2023-07-17 17:21:21.0802 PACKET    M1         #0x0001 AnswerRequestIdentification    > C0 (127.0.0.1:37966)
        2023-07-17 17:21:21.0804 PACKET    M1         #0x0000 NotifyNodeInformation          > C0 (127.0.0.1:37966)
        2023-07-17 17:21:21.0804 PACKET    M1          ! C0 | CLIENT  |                 | RUNNING | 2023-07-17 14:21:21.079838
        2023-07-17 17:21:21.0804 PACKET    M1          ! M1 | MASTER  | 127.0.0.1:21151 | RUNNING | None
        2023-07-17 17:21:21.0804 PACKET    M1          ! S1 | STORAGE | 127.0.0.1:40345 | RUNNING | 2023-07-17 14:21:18.737469
        2023-07-17 17:21:21.0805 PACKET    M1         #0x0002 NotifyPartitionTable           > C0 (127.0.0.1:37966)
        2023-07-17 17:21:21.0810 PACKET    M1         #0x0003 LastTransaction                < C0 (127.0.0.1:37966)
        2023-07-17 17:21:21.0811 PACKET    M1         #0x0003 AnswerLastTransaction          > C0 (127.0.0.1:37966)
        2023-07-17 17:22:06.2053 DEBUG     M1         <SocketConnectorIPv4 at 0x7f3584252d10 fileno 18 ('127.0.0.1', 21151), opened from ('127.0.0.1', 37966)> closed in recv
        2023-07-17 17:22:06.2056 DEBUG     M1         connection closed for <ServerConnection(nid=C0, address=127.0.0.1:37966, handler=ClientServiceHandler, closed, server) at 7f3584245910>
        2023-07-17 17:22:06.2058 PACKET    M1         #0x0014 NotifyNodeInformation          > A1 (127.0.0.1:37906)
        2023-07-17 17:22:06.2058 PACKET    M1          ! C0 | CLIENT |  | UNKNOWN | 2023-07-17 14:21:21.079838
        2023-07-17 17:22:06.2059 PACKET    M1         #0x0104 NotifyNodeInformation          > S1 (127.0.0.1:37918)
        2023-07-17 17:22:06.2059 PACKET    M1          ! C0 | CLIENT |  | UNKNOWN | 2023-07-17 14:21:21.079838

The problem is due to that my analysis from e407f725 (go/neo/neonet:
Rework handshake to differentiate client and server parts) turned out to
be incorrect. Quoting that patch:

    -> Rework handshake so that client always sends its hello first, and
    only then the server side replies. This matches actual NEO/py behaviour:

    https://lab.nexedi.com/nexedi/neoppod/blob/v1.12-67-g261dd4b4/neo/lib/connector.py#L293-294

    even though the "NEO protocol" states that

    	Handshake transmissions are not ordered with respect to each other and can go in parallel.

    	( https://neo.nexedi.com/P-NEO-Protocol.Specification.2019?portal_skin=CI_slideshow#/9/2 )

    If I recall correctly that sentence was authored by me in 2018 based on
    previous understanding of should-be full symmetry in-between client and
    server.

so here "This matches actual NEO/py behaviour" was wrong: even though
https://lab.nexedi.com/nexedi/neoppod/blob/v1.12-67-g261dd4b4/neo/lib/connector.py#L293-294
indeed says that

    #       The NEO protocol is such that a client connection is always the
    #       first to send a packet, as soon as the connection is established,

in reality it is not the case as SocketConnector always queues handshake
hello upon its creation before receiving anything from remote side:
https://lab.nexedi.com/nexedi/neoppod/blob/v1.12-93-gfd87e153/neo/lib/connector.py#L77-78 .
In practice this leads to that in non-SSL case NEO/py server might be
fast enough to send its prepared hello before receiving hello from us.
Levin also explains at kirr/neo!5 (comment 187429):

    I think what happens is this:

    the NEO protocol doesn't specify in which order handshakes happen after
    initial dial. If the peer sends a handshake before receiving our
    handshake and if this peers handshake is received by us, 'DialLink'
    assumes everything is fine (no err is returned), it breaks the loop and
    returns the link. But then, very little time later, when the peer
    finally receives our handshake, this looks strange for the peer and it
    closes the connection.

    So in my understanding this should be fixed by explicitly comparing the
    encodings between our expected one and what the peer provided us. If
    encodings don't match we should retry with a new encoding in order to
    prevent the peer from closing the connection.

    For me this also explains why sometimes the tests passed and sometimes
    didn't: it depended on which node was faster ('race condition').

-> In this patch we add correspondig handshake test that demonstrates
this problem. It currently fails as

--- FAIL: TestHandshake (0.01s)
    --- FAIL: TestHandshake/enc=N (0.00s)
        newlink_test.go:154: handshake encoding mismatch: client: unexpected error:
            have: <nil>
                 "<nil>"
            want: &neonet._HandshakeError{LocalRole:1, LocalAddr:net.pipeAddr{}, RemoteAddr:net.pipeAddr{}, Err:(*neonet._EncodingMismatchError)(0xc0000a4190)}
                 "pipe - pipe: handshake (client): protocol encoding mismatch: peer = 'M'  ; our side = 'N'"
    --- FAIL: TestHandshake/enc=M (0.00s)
        newlink_test.go:154: handshake encoding mismatch: client: unexpected error:
            have: <nil>
                 "<nil>"
            want: &neonet._HandshakeError{LocalRole:1, LocalAddr:net.pipeAddr{}, RemoteAddr:net.pipeAddr{}, Err:(*neonet._EncodingMismatchError)(0xc0001a22cc)}
                 "pipe - pipe: handshake (client): protocol encoding mismatch: peer = 'N'  ; our side = 'M'"

We will fix it in the next patch.

/reported-by @levin.zimmermann
/reported-on kirr/neo!5

go/neo/neonet: Dedicate an error type to indicate "protocol version mismatch" as handshake failure cause

We will soon need to detect if a handshake failure was due to mismatch
of protocol encodings and that would require introduction of dedicated
error type for that cause. As a preparatory step first refactor "version
mismatch cause" to follow the same style for symmetry.

This patch fixes a discrepancy between NEO/py and NEO/go: NEO/py expands
the '~' and the '~username' prefix in the file path of the TLS
certificate/key files [1]. This syntax is used in NEO/py SlapOS SR [2].
We need to fix this discrepancy in NEO/go in order to use TLS encryption
with NEO + WCFS.

[1] https://lab.nexedi.com/nexedi/neoppod/blob/7c539f0f/neo/lib/config.py#L149
and https://lab.nexedi.com/nexedi/neoppod/blob/fa63d856/neo/lib/app.py#L25-31

[2] https://lab.nexedi.com/nexedi/slapos/blob/397726e1/stack/erp5/instance-zodb-base.cfg.in#L18-20
and https://lab.nexedi.com/nexedi/slapos/blob/a8150a1a/software/neoppod/instance-neo-input-schema.json#L62

/reviewed-by @kirr
/reviewed-on !1

The xfilepath package supports resolving filepaths with a user prefix
to absolute paths: it converts '~' and '~username' to $HOME of user
(as it's done by for  instance bash). No builtin golang module supports this
functionality [1]. We need this functionality in order to imitate the
behaviour of NEO/py in NEO/go [2].

---

[1] https://stackoverflow.com/questions/47261719/how-can-i-resolve-a-relative-path-to-absolute-path-in-golang)
[2] nexedi/slapos!1307 (comment 17574)

/reviewed-by @kirr
/reviewed-on !1

Without parenthesis it was failing on py3:

	(neo) (py3.venv) (g.env) kirr@deca:~/src/neo/src/lab.nexedi.com/kirr/neo/go/neo/t$ ./neotest info-local
	date:   Wed, 18 May 2022 11:05:50 +0300
	xnode:  kirr@deca.navytux.spb.ru (2401:5180:0:af::1 192.168.0.3 (+ 1·ipv4))
	uname:  Linux deca 5.10.0-13-amd64 #1 SMP Debian 5.10.106-1 (2022-03-17) x86_64 GNU/Linux
	cpu:    Intel(R) Core(TM) i7-7600U CPU @ 2.80GHz
	  File "<string>", line 1
	    print '%.2fGHz' % (400000 / 1E6)
	          ^
	SyntaxError: invalid syntax

On py3 dict.keys() returns iterator instead of list:

    $ ./tzodb.py zhash
    Traceback (most recent call last):
      File "/home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/neo/t/./tzodb.py", line 141, in <module>
        main()
      File "/home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/neo/t/./tzodb.py", line 138, in main
        zhash()
      File "/home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/neo/t/./tzodb.py", line 59, in zhash
        optv, argv = getopt(sys.argv[2:], "h", ["help", "check=", "bench="] + hashRegistry.keys())
    TypeError: can only concatenate list (not "dict_keys") to list

With xurl.ParseQuery being simpler analog of url.ParseQuery.
Simpler: It returns regular map instead of url.Values by not allowing duplicates.

WCFS needs to know key coverage for every visited node.

WARNING: this is API change.

KeyRange represents [lo,hi) key range. It simplifies working with ranges
of keys. We will use it in the next commit.

KeyRange originated in WCFS and was copied from there:

https://lab.nexedi.com/kirr/wendelin.core/blob/57be0126/wcfs/internal/xbtree/blib/keyrange.go

We were already using math.Min<Key> in one place, but the number of such
places is going to increase. -> Keep min/max definition in only one place.

NOTE: db was already being closed in the test's code.

staticcheck says:

	xtesting.go:386:2: this value of err is never used (SA4006)

fs1/format.go:204:2: only the first constant in this group has an explicit type (SA9004)
zeo/proto.go:56:2: only the first constant in this group has an explicit type (SA9004)

staticcheck reports

    ziobtree.go:606:4: Errorf is a pure function but its return value is ignored (SA4017)
    ziobtree.go:626:4: Errorf is a pure function but its return value is ignored (SA4017)
    zlobtree.go:606:4: Errorf is a pure function but its return value is ignored (SA4017)
    zlobtree.go:626:4: Errorf is a pure function but its return value is ignored (SA4017)

run_with_zodb3py2_compat was renamed to run_with_zodb4py2_compat in
nexedi/zodbtools@c59a54ca .

Without the fix go genrate was failing as

    (neo) (z-dev) (g.env) kirr@deca:~/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/btree$ go generate
    Traceback (most recent call last):
      File "./testdata/gen-testdata", line 26, in <module>
        from zodbtools.test.gen_testdata import run_with_zodb3py2_compat
    ImportError: cannot import name run_with_zodb3py2_compat

and

    (neo) (z-dev) (g.env) kirr@deca:~/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb$ go generate
    Traceback (most recent call last):
      File "./py/pydata-gen-testdata", line 24, in <module>
        from zodbtools.test.gen_testdata import run_with_zodb3py2_compat
    ImportError: cannot import name run_with_zodb3py2_compat

This amends commit fc69e00d (go/zodb/fs1: Fix Python database generator
to work with recent zodbtools).

- the only valid range for at is [tail, head]. Don't try to return anything
  meaningful for queries outside of this range and just panic instead. This is
  consistent with SliceByRev, which also panics on invalid query, and it is also
  consistent with semantic model that ΔTail is a vector with data keyed by tid in
  range (tail, head]: if key is out of vector range, access to the vector should
  panic, isn't it?

- instead of returning revision of minimum entry on exact=n, always
  return (tail, exact=n) in that case.

The change in behaviour is consistent with ΔFtail and ΔBtail from WCFS
and is needed for ΔFtail to function correctly:

https://lab.nexedi.com/kirr/wendelin.core/blob/22f5f096/wcfs/internal/xbtree/δbtail.go
https://lab.nexedi.com/kirr/wendelin.core/blob/22f5f096/wcfs/internal/zdata/δftail.go

When object is just created, it is not yet assigned an OID, but can be
reachable from other objects. The code that processes transaction can
reach to that new object and try to PActivate/PDeactivate it. And
currently PDeactivate will drop the object state completely.

Another example of object without an OID is Bucket embedded into a Tree
object. There, the code that scans the tree can reach to that bucket and
try to activate/deactivate it, leading, again, to dropping state of that bucket.

-> Fix it.

Persistent.PActivate used to panic when called the second time, if the
first time it hit an error. WCFS hit this in practice via object, that was
previously accessed and pinned into the cache, but later deleted in the
storage.

-> Fix PActivate to reset .loading on an error, so that next time PActivate is
called, it tries to trigger load again instead of panicking. Change doload
criteria from

	state==GHOST && refcnt==1

to

	state==GHOST && loading==nil

because now, after failed PActivate, refcnt can be != 0, if there are several
other PActivate calls that were waiting for the failed PActivate but did not
yet woke up.

Here is how added test fails without the fix:

    --- FAIL: TestActivateAfterDelete (1.65s)
    panic: t.zodb.MyObject(0000000000000065): activate: need to load, but .loading != nil [recovered]
            panic: t.zodb.MyObject(0000000000000065): activate: need to load, but .loading != nil

    goroutine 10085 [running]:
    testing.tRunner.func1.2(0x649020, 0xc000520660)
            /home/kirr/src/tools/go/go/src/testing/testing.go:1143 +0x332
    testing.tRunner.func1(0xc0001cb080)
            /home/kirr/src/tools/go/go/src/testing/testing.go:1146 +0x4b6
    panic(0x649020, 0xc000520660)
            /home/kirr/src/tools/go/go/src/runtime/panic.go:965 +0x1b9
    lab.nexedi.com/kirr/neo/go/zodb.(*Persistent).PActivate(0xc0001184d0, 0x6e8360, 0xc00019ac90, 0x0, 0x0)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/persistent.go:191 +0xce5
    lab.nexedi.com/kirr/neo/go/zodb.TestActivateAfterDelete(0xc0001cb080)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/persistent_test.go:786 +0x72c

Add utility to verify FileStorage data for consistency.
To verify we just need to iterate through all records, because
FileStorage driver performs all consistency checks by itself.

Mimic normal output to be the same as in fstest from ZODB/py.
Example runs of fstest.py and `fs1 verify` on a broken file:

    $ python ~/src/wendelin/z/ZODB/src/ZODB/scripts/fstest.py -v 1.fs
             4: transaction tid 0x03e044f6448c8022 #0
           213: transaction tid 0x03e044f646e044bb #1
    1.fs has data records that extend beyond the transaction record; end at 466

    $ fs1 verify -v 1.fs
             4: transaction tid 0x03e044f6448c8022 #0
           213: transaction tid 0x03e044f646e044bb #1
    2021/05/24 12:43:37 fsverify: 1.fs: 1.fs: transaction record @355: -> (iter data): 1.fs: data record @416: check: data record [..., 466) overlaps txn boundary [..., 458)

As can be seen, in fs1 case, the error contains more details: [start, end) of
both mismatching transaction and data records.

In addition to fstest-like verbosity, add progress-mode, where % of total
completion is printed in a style similar to one used by `fs1 verify-index`.

The Go-based implementation is also faster even when data is on HDD. For
example on a 73GB database provided by @jerome[1] fsrefs.py takes ~15 minutes
to run and occupy ~70-100% of CPU. On the other hand `fs1 verify` takes ~7
minutes to run and occupy ~ 20-30% of CPU.

Tests pending.

[1] nexedi/zodbtools!19 (comment 129480)

Some dumpers might want to print something at the end of their dump.
We will need this functionality for Verify (see next patch).

And use that in the callers.

This error type is documented (see Loader) to be always created as
*NoObjectError.

-> Fix Error receiver accordingly.

WCFS needs this to run tests faster.

In general it is also a good idea to pass options to DB constructor, in
particular options that affect live cache size, or other properties, for
further created connections.