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.)

Add a test of the new iterator() feature in TransactionalUndoStorage.