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.