- 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.

For Load demo.Storage implementation is similar to DemoStorage in
ZODB/py with fixes "cherry-picked" from:

- https://github.com/zopefoundation/ZODB/issues/318
  (DemoStorage does not take whiteouts into account -> leading to data corruption)

- https://github.com/zopefoundation/ZODB/pull/323
  (loadAt + fix for the above issue)

For safety demo.Storage - contrary to DemoStorage/py - actually verifies
that for demo=base+δ δ comes strictly after base and that base remains
unchanged.

URI schema follows XRI Cross-references approach and is

	demo:(zurl_base)/(zurl_δ)

https://en.wikipedia.org/wiki/Extensible_Resource_Identifier provides
some related details and examples.

For ZODB/py corresponding pull-request for zodburi to support demo: URI
scheme has been made here: https://github.com/Pylons/zodburi/pull/29 .

Tests need:

- recent zodbtools with zodbrestore:

  https://lab.nexedi.com/nexedi/zodbtools/blob/129afa67/zodbtools/zodbrestore.py
  nexedi/zodbtools!19

- ZODB with support for DemoStorage.deleteObject
  https://github.com/zopefoundation/ZODB/pull/341

On Go side demo storage is needed for wendelin.core 2 because ERP5 uses
DemoStorage to run tests.

This is low-level API to open IStorageDriver instead of IStorage.
Demo storage will need this.

Maybe it would be a good idea to move drivers-related functionality into
separate package in the future.

In ZODB/go when there is no schema in zurl, open automatically prepends
file:// . However filename itself could contain ":" and so generally
speaking it is incorrect to return URL without file:// schema prepended
to file name.

Another reason to always use fully-constructed URLs with schema, is
interoperability with ZODB/py - there zodburi, when given zurl without
schema, does not make any assumption that it is of file:// kind and
rejects opening such URIs.

An URI schema is required to have ":" after it, but - even if frequently
used in practice - not //. We will soon introduce "demo:" URI scheme
that comes without //, so fix Open to detect schema presence just by ":"
and not to fixup "demo:..." url to "file://demo:..." automatically.

Before the patch if storage.watcher fails, storage.driver.Close is not
called, and so the driver will continue to send to .drvWatchq, but noone
is receiving from it.

a5dbb92b (go/zodb: Require drivers to close watchq on Close), provides
the guarantee that the driver will stop sending on drvWatchq right after
drv.Close call.

Provide guaranty that Close forces the driver to stop sending to watchq
and to close it. See a5dbb92b ("go/zodb: Require drivers to close watchq
on Close") for details.

Without the fix TestWatch fails with test timeout:

    panic: test timed out after 30s

    # Close waits for serve to stop
    goroutine 93 [semacquire]:
    sync.runtime_Semacquire(0xc000152170)
            /home/kirr/src/tools/go/go/src/runtime/sema.go:56 +0x45
    sync.(*WaitGroup).Wait(0xc000152168)
            /home/kirr/src/tools/go/go/src/sync/waitgroup.go:130 +0x65
    lab.nexedi.com/kirr/neo/go/zodb/storage/zeo.(*zLink).Close(0xc0001520f0, 0x1313, 0x1)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/storage/zeo/zrpc.go:159 +0x47
    lab.nexedi.com/kirr/neo/go/zodb/storage/zeo.(*zeo).Close(0xc000313680, 0xc000107c78, 0x1)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/storage/zeo/zeo.go:526 +0x2e
    lab.nexedi.com/kirr/neo/go/internal/xtesting.DrvTestWatch(0xc000082c00, 0xc0000aa2a0, 0x24, 0x6a4a38)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/internal/xtesting/xtesting.go:442 +0xdb5
    lab.nexedi.com/kirr/neo/go/zodb/storage/zeo.TestWatch.func1(0xc000082c00, 0x6e3498, 0xc00009a380)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/storage/zeo/zeo_test.go:270 +0x99
    lab.nexedi.com/kirr/neo/go/zodb/storage/zeo.withZEOSrv.func2.1(0xc0000a4168, 0x16)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/storage/zeo/zeo_test.go:207 +0xfb
    lab.nexedi.com/kirr/neo/go/zodb/storage/zeo.withZEOSrv.func1(0xc000082c00, 0xc00009c180)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/storage/zeo/zeo_test.go:186 +0x129
    lab.nexedi.com/kirr/neo/go/zodb/storage/zeo.withZEOSrv.func2(0xc000082c00)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/storage/zeo/zeo_test.go:199 +0x10e
    testing.tRunner(0xc000082c00, 0xc00009c160)
            /home/kirr/src/tools/go/go/src/testing/testing.go:1194 +0xef
    created by testing.(*T).Run
            /home/kirr/src/tools/go/go/src/testing/testing.go:1239 +0x2b3

    # serve is stuck in invalidateTransaction doing watchq<-
    goroutine 26 [chan send]:
    lab.nexedi.com/kirr/neo/go/zodb/storage/zeo.(*zeo).invalidateTransaction(0xc000313680, 0x6417e0, 0xc000323b60, 0x0, 0x0)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/storage/zeo/zeo.go:176 +0x373
    lab.nexedi.com/kirr/neo/go/zodb/storage/zeo.(*zLink).serveRecv1(0xc0001520f0, 0xc000393890, 0x0, 0x0)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/storage/zeo/zrpc.go:225 +0x4b4
    lab.nexedi.com/kirr/neo/go/zodb/storage/zeo.(*zLink).serveRecv(0xc0001520f0)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/storage/zeo/zrpc.go:176 +0x8d
    created by lab.nexedi.com/kirr/neo/go/zodb/storage/zeo.(*zLink).start
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/storage/zeo/zrpc.go:99 +0xc8

Provide guaranty that Close forces the driver to stop sending to watchq
and to close it. See a5dbb92b ("go/zodb: Require drivers to close watchq
on Close") for details.

If we don't require drivers to stop sending to watchq after Close, there
could be many deadlock scenarios, for example:

- client called drv.Close(); no longer listens to watchq; driver is
  stuck sending to watchq, or

- client called drv.Close(), which itself waits for tasks spawned inside
  driver to complete, which are stuck sending to watchq because client
  no longer does <-watchq.

The change is similar in spirit to safety guaranty provided by high-level
Watcher where after DelWatch call it is guaranteed that there will be no
more sends to subscribed watchq (see c41c2907 "go/zodb: High-level
watching - initial draft") for details.

All drivers don't provide requested guarantee yet.
We'll be fixing them one-by-one in followup commits.

Else, on an error, it is the lineno of `t.Fatal(err)` inside FatalIf
that is printed, not the line number inside user test.

A data record with len(data)=0 and backpointer=0 is considered by
FileStorage/py as "no data":

https://github.com/zopefoundation/ZODB/blob/5.6.0-15-g22d1405d4/src/ZODB/FileStorage/FileStorage.py#L576-L582

Even though currently it is not possible to create such data record via
FileStorage(py).deleteObject (becase it raises POSKeyError if there is
no previous object revision), being able to use such data records is
semantically useful in overlayed DemoStorage settings, where δ part
marks an object that exists only in base with delete record whiteout.

It is also generally meaningfull to be able to create "delete" record
even if object was not previously existing: "deleteObject" is actually
similar to "store" (and so should be better named as "storeDelete"). If
one wants to store deletion, there should not be a reason to reject it,
because deleteObject already has seatbelt in the form of oldserial, and
if the user calls deleteObject(oid, oldserial=z64), he/she is already
telling that "I know that this object does not exist in this storage
(oldserial=z64), but still please create a deletion record for it". Once
again this is useful in overlayed DemoStorage settings described above.

For the reference, such whiteout deletion records pass ZODB/scripts/fstest just fine.

Even though FileStorage/py loads such data records just fine, on
FileStorage/go side it was not the case - DataHeader.LoadBackRef, even
with backpointer=0, was verifying that backpointer to be valid and
failing seeing it might overlap with current transaction:

    === RUN   TestLoadWhiteout
    2021/03/17 06:40:58 index load: open testdata/whiteout.fs.index: no such file or directory
    2021/03/17 06:40:58 testdata/whiteout.fs: index rebuild...
        filestorage_test.go:398: load 0000000000000017:0000000000000001: bad err:
            have: testdata/whiteout.fs: load 0000000000000017:0000000000000001: testdata/whiteout.fs: data record @27: check: backpointer (0) overlaps with txn (4)
            want: testdata/whiteout.fs: load 0000000000000017:0000000000000001: 0000000000000001: object was not yet created

It was a thinko: backPos==0 was already kind of handled in LoadBackRef,
but only in one verification case. -> Fix all checks not to trigger when
seeing backPos=0. DataHeader.LoadBack - the caller of LoadBackRef -
already handles returned backPos=0 as "no data".

We were reusing Dumper instance in between testDump subtests. This was
not noticed, as it was only "1" and then "empty" case, because "emtpy"
has no transactions. However in the next patch we'll add another
subcase, and if the dumper instance is not reset, it will think that it
starts from transaction number non-zero, which would differ from fresh
dumper output.

-> Fix it.

As Zodbtools dropped ZODB3 support its run_with_zodb3py2_compat was
renamed to run_with_zodb4py2_compat:

nexedi/zodbtools@c59a54ca

Encode/decode was deprecated and removed in recent github.com/shamaton/msgpack.

NEO/py switched from UUID to 4-byte NodeID long time ago in
nexedi/neoppod@23fad3af and has a TODO to
rename uuid to nid.

NEO protocol also talks about renaming uuid to nid:

    note: the uuid variable should be renamed into nid
    https://neo.nexedi.com/P-NEO-Protocol.Specification.2019?portal_skin=CI_slideshow#/9/5

NEO/go can do the rename now.

It is the same as regular SHA1 but returns all-zeros for empty input.
Besides there is runtime knob to skip checksum computation to benchmark
how much time and latency SHA1 computation introduces.

See the following links for some preliminary history:

- bc2cddfc
- https://navytux.spb.ru/~kirr/neo.html#results-and-discussion

See previous commits.

To use something like

	defer task.Running(&ctx, "my task")(&err)

that will create the task, trace its begin/end and integrate with return
error.

Probably this package should be integrated one way or another with
go/internal/xcontext/task.

Package log provides logging with severity levels and integration with tasks.

Package task provides primitives to track tasks via contexts.

NEO/go server accepts preferred encoding of client from start, but
NEO/py server implements either N or M encoding and disconnects client
silently if handshake is not exactly what is expected.

However we can teach Dial to retry with different preferred options and
this way when connection to a NEO/py server, it will essentially
autodetect which encoding is used and accepted by remote peer.

This patch adds support for serializing packet frames with M encoding on
the wire. To do so it follows rules defined in

    	nexedi/neoppod@9d0bf97a
    	( nexedi/neoppod!11 )

Server handshake is reworked to autodetect client's preferred encoding.
Client always prefers 'N' for now.

This patch adds proto.Encoding('M') and teaches it to encode/decode
messages via MessagePack by the rules defined in

	nexedi/neoppod@9d0bf97a
	( nexedi/neoppod!11 )

It only adds support for messages serialization, without changing
proto.go to match changes in e.g. enums reordering, and without adding
support for MessagePack at link layer in neonet.

M-encoding was only tested for NEO/go-NEO/go, and was not yet tested for
NEO/go-NEO/py interoperation. There will be likely small mistakes
discovered on my side that should be hopefully easy to fix step by step
once we get to that phase.

It complements github.com/tinylib/msgp with things that neo/proto needs
to encode/decode messages.

It is noop for 'N' encoding, but will be non-empty for MessagePack.

genSliceHead and genMapHead will be reused for MsgPack encoding.
Changes in generated code are mostly due to move of where things are
typecasted to e.g. uint32 without any change in semantic.

For decode-family of functions protogen.go sticks to convention to name
target variable to where to assign the result as "assignto", not "path".

No change in generated code.

Encoding specifies a way to encode/decode NEO messages and packets.
Current way of how messages were encoded is called to be 'N' encoding.

This patch:

- adds proto.Encoding type
- changes MsgEncode and MsgDecode to be methods of Encoding
- renames thigs that are specific to 'N' encoding to have 'N' suffix
- changes tests to run a testcase agains vector of provided encodings.
  That vector is currently only ['N'].

And provide only single top-level entry-points to encode/decode
messages. As of now the entry points are just plain forwarding, but with
introducing of msgpack and encodings, they will take into account
through which encoding a message has to be encoded/decoded.