Implement invalidation handling to teach DB to reuse Connections from
connection pool: after connection is returned to the pool on transaction
completion, we can use this connection for next DB.Open(at) request, by
seeing which objects we changed in conn.at..at range and invalidating
those objects in connection live cache.

To know which objects were changed, DB adds watch on its storage and
maintains some history tail (using ΔTail - see previous commit).

Finally add test for both DB and Connection, and also for Persistent,
LiveCache, ... - as all those application-level components are tightly
inter-related.

Add ΔTail - utility class to keep track of history tail of revisional
changes. This will be needed for both DB (to handle invalidations) and
for raw Cache(*). It also might be useful in places outside of zodb/go -
for example WCFS from Wendelin.core uses ΔTail to keep track of which
ZODB revision changed which block of file(+).

Please see ΔTail comments for details.

(*) to check a bit ahead on load to see whether loaded serial is
actually the latest one for ≤ Cache.head, and, if yes, mark
corresponding cache entry as having .head="current" so that the entry
coverage could be extended when Cache.head is extended to cover more
database state instead of loading the same data from the database again.

(+) that's why we have δtail.go.cat-generic, so that third-party
packages could adapt ΔTail to needed types.

We are going to add more options. Adopt "!" prefix as "no" for
something. Make the code flow in ConnOptions more suitable for further
additions without changing formatter for previous option.

DB is a "handle to database". It has to be closed to release its
resources when no longer needed. We don't have to release anything
currently, but that will soon change in a followup patch.

Due to forgotten ... it was not passing prepared argv as argv to
fmt.Errorf, but as a single slice argument. The result was messy, for
example

    persistent_test.go:176: deactivate refcnt < 0: panic error unexpected:
        have: "[t.zodb.MyObject 000000000000000c](%!s(MISSING)): deactivate: refcnt < 0"
        want: "t.zodb.MyObject(000000000000000c): deactivate: refcnt < 0"

Using the occasion add details to deactivate refcnt panic, which
made me to discover this bug.

Seeing all problems helps to understand what is going on, compared to
e.g. if it complains only about .jar being wrong.

Persistent.refcnt is int, %s prints it as e.g.

	%!s(int=1)

-> use %d.

We are going to use this function in other tests too.

We were not actually checking for it and it is hard to check correctly
since .loading is transient.

Add new `zodb watch` command that is both demonstration of just added
ZODB Watch API (see previous patch), and could be useful for a ZODB
database administration / monitoring.

See zodbtools/watch.go for description.

Add initial draft for IStorage-level watch API and implemention:

- Notifier goes away;
- Add Watcher interface instead with AddWatch/DelWatch calls. See
  Watcher documentation for details.
- IStorage wrapper now subscribes to driver events, retranslate them
  to users and handles Add/Del watch subscription requests.

XXX Raw cache becomes temporarily disabled, until it is fixed to handle invalidations.
XXX Tests pending.

storage is a wrapper that turns IStorageDriver into IStorage. It will
have some logic around almost all IStorageDriver methods. So don't
automatically inherit driver methods not to miss some.

It is already not only open code there, and we are going to add more
storage wrapping functionality into storage (IStorage wrapper over
IStorageDriver). Storage.go thus seems to be a better name.

This continues 4d2c8b1d (go/zodb: Require drivers to provide
notifications for database change events) and a6580062 (go/zodb: Require
drivers to provide at₀ on open) and reworks watchq to receive not only
commit events, but also errors from watcher in storage driver.

Similarly to zodb (632c9798 "go/zodb: storageURL -> zurl")

This are equivalents of PersistentMapping and PersistentList in ZODB/py.
PersistentMapping is usually used as root ZODB object in Zope/ERP5/...

At least we should be able to load and save them now.
Tests pending...

zurl is more handy and is also used in zodbtools which have help on it
(`zodb help zurl`).

BTrees.Length.Length is used in many places in ERP5. At least we should
be able to load and save it now.

Tests pending.

This functions invoke visit callback on every BTree node they traverse.

Wendelin.core needs this to retrieve serials of not only looked up ZBlk
object, but also of every BTree nodes on the path to it.

go/zodb/btree: Bucket.Get: Don't get ctx argument and just require that the bucket must be already activated

It was scheduled to be so as described in removed TODO.

It is either BTree or Bucket.

This will allow persistency layer to do kind of copy-on-write, if e.g.
there are many connections that are sharing object's state.

Continuing previous patch also teach ZEO to decode identifies from
on-wire as both [8]str or [8]byte.

Tests pending.

Since ZODB 5.4 OID is encoded as bytes, not str:

	https://github.com/zopefoundation/ZODB/commit/12ee41c473

Try to support both str and bytes on decoding, as we need to support all
ZODB versions.

Add a way for tests to serialize and commit IPersistent objects. The
commit, for now, is implemented via calling external zodbtools/py to
commit raw data (see 7f14e2cb "go/zodb/fs1: tests: Factor-out commiting
a transaction via ZODB/py into common place). The data to commit is what
object serialization (see previous patch) gives.

For now there is no tracking of dirtiness. A test must explicitly list
objects it wants to save into database.

This functionality will soon be used to implement Persistent/DB/...
tests.

This will be used in the future to commit object changes back into
database. No test, since this functionality will be used at first to
implement tests themselves (see next patch). Hopefully after
bootstrapping we'll have full tests.

- require (Py)GetState and make it clear that it is called by
  persistency machinery only on non-ghost objects.
- make it clear that (Py)SetState is called by persistency machinery
  only on ghost objects.
- make it clear that DropState is called by persistency machinery only
  on non-ghost objects.

For btree PyGetState is marked as TODO which we'll fill incrementally
(the code is draftly ready, but there is no test for now).

We will soon need to be able to commit in tests for ZODB itself.

and to deliver to watchq only and all events in (at₀, +∞] range.

This continues 4d2c8b1d (go/zodb: Require drivers to provide
notifications for database change events) and makes initialization
semantics to process invalidations easier for both application-level ZODB
layer, and for low-level users that work with drivers directly:

- there is no possibility that an event will come from watchq with tid <
  current user head.

- there is no possibility that watcher will start delivering events
  after at₀, but not immediately after it, i.e. users can rely that they
  won't loose an event.

This correctness invariants should be easy to provide in drivers.

pool is a better name for "pool of unused connections" compared to connv.

Connection has .db and db has .stor - there is no need to keep separate
.stor on the Connection. This thinko was there from Connection beginning
(533f0c73 "go/zodb: DB - application-level handle to database (very draft)")

There was a thinko in transaction.Abort - it was spawning synchronizers
under a waitgroup, but wg.Wait() call was forgotten. This way, e.g. in
ZODB if a transaction was aborted, corresponding connection could be not
yet returned back into DB pool.

Fix it.

Test is TODO for the time when, hopefully, tracetest is generally ready.

In ZODB transaction ID is connected with time. We already have
functionality to convert tid to time (see bac6c953 "go/zodb: Tid
connection with time"), but the functionality for converting in another
way - time -> tid - was missing.

Fix it.

It is useful in situations where one need to print potentially Broken
objects properly, because %T does not print that detail and using %#v
creates too much noise.

This allows for applications to tune eviction strategy for their needs.
The interface to do so (LiveCacheControl) was defined previously in
c67ff9ea (go/zodb: Connection: Allow applications to tune live-cache
eviction policy), but so far there was no way to install cache
controller. Fix it.

Wendelin.core (wcfs) needs to check whether an object is currently
cached or not.

Following-up on 4d2c8b1d (go/zodb: Require drivers to provide
notifications for database change events) let's teach FileStorage to
support watching for database changes:

- add watcher who observes changes to data file via fsnotify.

- when we see that the file was updated - there is a tricky case to distinguish
  in-progress writes from just some garbage added at tail. See comments in
  _watcher for details.

- if the watcher sees there is indeed new transactions, the watcher
  updates index & txnh{Min,Max} and sends corresponding event to watchq.

- since index / txnh{Min,Max} can now be changed (by watcher) -> they
  are now protected by mu.

- consequently operations like LastTid, LastOid, Load, ... are all taught to
  observe index / txnh{Min,Max} state in read-snapshot way.

- add down & friends to indicate that the storage is no longer
  operational, if watcher sees there is a problem with data file.

- Open is reworked to start from loading/rebuilding index first, and then tail
  to watcher to detect whether what's after found topPos is garbage or another
  in-progress transaction. Consequently it is now possible to correctly open
  filestorage that is being currently written to and has in-progress transaction
  at tail.

The patch is a bit big, but the changes here are all tightly
interrelated.

It is already 3 places where we return zodb.OpError and it will be more.
Make a helper function to create those errors with higher signal/noise.

A ZODB database can be changed by local client as well as another
process. A notification channel is thus needed for local cache and
database view to know they have to update to current database state.

This patch builds the interface of how such notifications should be
provided by drivers. Contrary to ZODB/py it is required that every
driver provide it.

However we will be providing driver support incrementally and for now
all drivers behave as if the database is not changing.

A note on why Watchq is passed to driver as options: low-level ZODB
users, who might want to work with drivers directly, might not need it,
and this way with Watchq not present in driver options they will continue to
observe the same driver behaviour as before watchq was introduced. In
practice many low-level utilities don't need notification support, and
it would be not good to required them all to update open calls and to
provide watchq drainer not to get stuck.

TODO Watchq should be extended to also receive errors from watcher, so
that clients could be notified when there is something wrong with the
database.