storage speak, lrevid pointers to shared pickle data in earlier
transactions.

Specifically,

The _Record object now has a data_txn attribute that is either None or
the id of the transaction that contains the data used by the current
record.  Example: When transactionalUndo() modifies an object, it
typical creates a new data record that points at the transaction
before the undo.  The new record contains the same logical data as the
record it refers to.  (For consistency purposes, this is a stronger
claim than that the pickles in two different data records are the
same.)

_Record.__init__(): Gets data_txn passed in from
_RecordIterator.__getitem__(), which in turn getes the lrevid from
Full._loadSerialEx().

_loadSerialEx(): Now returns the backpointer, aka lrevid but only if
it is not the same as the serial argument.  They will always be the
same (and we'll always return None) except in the face of
transactionalUndo, commitVersion, and abortVersion.