This spares clients from having to do it themselves repeatedly.
A dubious consequences is that Transaction.__init__ now requires a
WeakSet of synchronizer objects (or None -- so it's not a
catastrophic requirement, just irksome).

Weak sets have have pragmatic gotchas, explained in the comments
before the new WeakSet.as_weakref_list() method.  In essence, we
just took all the weak sets of connection objects and changed
everything so that a list of live objects is never materialized
anymore.  Also added new map()-like methods so that clients don't
usually need to be aware of the weakrefs under the covers.

This can't use self._connectionMap, because it needs to
know the version associated with each connection.

that the trunk is no longer strongly related to 3.3
development (the later takes place on the 3.3 branch).

There's no longer a hard limit on # of open connections per DB.

Introduced a sane scheme for raising deprecation warnings.
Sane ==

1. The machinery ensures that a "this will be removed in ZODB 3.6"
   blurb gets attached to all deprecation warnings.

and

2. It will dead easy to find these when it's time for 3.6.

Code cleanups.

Imports normalization.

The tests here have to be run manually for now, so rename
the directory so test.py doesn't complain about it.

Merge revs 27788 and 27795 from Zope trunk.

See Collector #1350.

ghostify() and unghostify() detect one form of thread insanity
now.  In a debug build, they abort the process if it happens.
In a release build, unghostify() raises SystemError, and
ghostify() ignores it (ghostify() can't raise an exception).

Use proto 1 pickles in ZEO/zrpc/Marshaller.encode().

This rehabilitates the good part of Andreas's change; adds
news; adds comments; and adds more comments about the
sequence of undocumented cPickle gimmicks this relies on.

Merge rev 27722 from Zope trunk.

Collector #1517: History tab for ZPT does not work

FileStorage.history() was reading the user, description,
and extension fields out of the object pickle, due to
starting the read at a wrong location.

Merge rev 27712 from Zope trunk.

Restore the Connection._opened attribute, actually used by
DB.connectionDebugInfo().

Transaction.begin().

When raising DeprecationWarning, point the warning machinery at
the caller, not at Transaction.begin() itself.  As is (voice
of experience <wink>), these warnings are darned near useless to
track down code that needs to be changed on Zope trunk.  With the
change, the warnings point directly at the deprecated uses.

Check in stuff mistakenly committed from the Zope trunk.

Rev 26842 changed cache-size parameters to type byte-size.
That's a good change, but it was checked in from a wrong place
(from the Zope trunk's copy of ZODB).  Since we want Zope 2.8 to
be usable with ZODB 3.3 too, I need to slam this change into
3.3 now.

Alas, I don't understand how ZODB+ZEO got stitched into the
Zope trunk (I do understand how it got stitched into the
Zope3 trunk, and into the X3 branch), and it looks like that
was left in a poor state.

New test checkReadConflictErrorClearedDuringAbort().

This confirms that 3.3 didn't have the 3.2.4c1 bug wherein
an explict transaction abort() failed to clear Connections'
memory of ReadConflictErrors.

Act as if a 3.3c1 release were being made.  Maybe it will
be.  Besides "the usual" release fiddling, repaired many
out-of-date pieces of the ZODB programming guide; I'm sure
many remain, though.

A test here didn't clean up after a failing commit().

Removed obsolete comment.

As discussed on zodb-dev, failing commit "sticks" now.

After a commit fails (raises an exception), all subsequent attempts
to commit, join, or register with the transaction now raise the new
TransactionFailedError.  The failed transaction must be explicitly
discarded now, via abort() on the transaction or begin() on its
transaction manager.

Officially deprecate Transaction.begin().

Forward port from Zope 2.7 branch.

The ConflictError.get_{old,new}_serial() methods each did what the other
was supposed to do.

Port from Zope 2.7 branch.

Collector #1488 (TemporaryStorage -- going backward in time).

This confusion was really due to that the detail on a ConflictError
exception didn't make sense.