_dorestore(): Adopt the FileStorage semantics for when the transaction
pointed to by prev_txn does not exist.  This can happen if we're
restoring to a storage where that transaction has been packed away.
In that case, treat prev_txn as a hint and ignore it. :)

_TransactionsIterator.__len__(): Give this a dummy (and lying)
implementation which is just good enough for Python 2.1.3 support.