More code and comment cleanups.  Notable changes:

FileCache:  removed the currentsize attribute.  It was initialized
to 0 but never updated.  Some tests referenced it, but since it was
always 0 those tests weren't getting any good from it.  Don't
recall (or never knew) what its intended purpose may have been.

FileCache.remove():  Repaired major bug.  This mistakenly stored
the "this disk block is free now" status byte into the start of
the serialized Object record's end_tid field (incorrect seek
offset).  The in-memory structures were correct then, but got out
of synch with the disk file; the latter then still claimed to have
info for a "live" object revision, but that revision was actually
dead, and the info on disk had a corrupt end_tid value.

Many small comment and code improvements.

Explicitly cast to float.  Else MSVC generated
legit warnings everywhere this macro was used,
warning about possible silent precision loss.

This adds a new IFBTree type, intended for use in indices.  Still
needs NEWS, still needs docs, and there are a pile of legitimate
compiler warnings on Windows that need fixing.

Forward port from ZODB 3.2.

Collector 1581:  fspack can blow up when .fs is corrupted

Repaired three places where fspack referenced an undefined global while
*trying* to raise CorruptedError.  Added new checkCorruptionInPack()
test to verify the correct exception gets raised.

Change ConflictError constructor to stop importing app objects.

When the ConflictError constructor is passed a pickle, extract
the module and class names without loading the pickle.  A ZEO
server doesn't necessarily have the implementation code for
application classes, and when that's so the attempt to raise
ConflictError was itself dying with an ImportError.

This one doesn't exist on the 3.3 branch.

Move more get_pickle_metadata imports.

Import get_pickle_metadata from its new home.

Move get_pickle_metadata() into utils.py.

Try to make more sense of the ZODB pickle format "docs".

Try to get ring.h installed.

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.