Add support for persistent weak references.  The implementation is in
pure Python right now; coptimizations.c was disabled.  We need to
restore the C code before going to beta.

The persistent reference format has evolved a little, but the code on
the branch doesn't document it.  The ref is usually a an oid-class
pair, where the class is an actual class object.  It can also be a
list, for weak references, or a bare oid.

Add support for data managers written against the ZODB4 transaction
API.  The transaction implementation provides an adapter between the
two interfaces.  There's a substantial doctest test suite for this
feature.

Add add() method to Connection to explicitly add an object and get it
an assigned _p_oid.  It's possible to add an unconnected object; this
isn't recommended, because it will be lost at pack time.

 port we plan to listen on.

Missed two tests in earlier checkins.

The old undo was not used by Zope and shouldn't have been used by any
other client.  The newly named undo() is the preferred version of
undo.

XXX DemoStorage didn't implement transactionalUndo, so now it doesn't
implementation undo() at all.  We need to replace it with the demo
storage from ZODB4.

There are a few changes related to ZODB4 removal in this checkin.

I've removed all the details about Python versions prior to 2.3 and
Berkeley before 4.1.25.  I also removed all the configuration
instructions.  You "just" use the standard zconfig mechanism, which
may or may not be documented elsewhere.

and set it in more places

an older ZConfig API which has not been supported for some time

I think it's safe to lose the revision history for these files.
setup.py is a copy of ZODB3/setup.py with all the paths edited.
test.py is a copy of ZODB4/test.py with no changes.

trouble here on Windows, due to the poor granulatiry of Windows time.time()
feeding into pack-time selection in this test.  Moved the snooze() from
before pack time selection to after it, which appears to be a sufficient
fix; this can't hurt Linux, because the snooze() calls aren't needed at
all there (but don't hurt either).

for a redundant pack.

"Yow!" here anymore <wink>.

Also remove some unused imports and a somewhat irrelevant comment.

I don't really understand why this change and the last one to
runzeo.py have their apparent effects, but the tests work and the -m
option to runzeo.py works.

Noted by Jon Dyte.

Bug fix from Jon Dyte.

The comment long ago fell out of synch with the code.

The Connection class inherits from the ExportImport mixin, but they
had an arm's length interaction.  Restructure the code to provide
direct interaction via the _import attribute.

Remove the more general onCommitCallback() method.  It was only used
by ExportImport.

A lot of cosmetic changes to ExportImport:
    - don't make local variables out of attribute lookups
    - avoid local variable names that shadow builtins
    - prefer isinstance() to type comparisons
    - prefer absolute imports to relative imports