QPID-4327: HA clean up transaction artifacts at end of TX.

- Backups delete transactions on failover.
- TxReplicator cancel subscriptions when transaction is finished.
- TxReplicator rollback if destroyed prematurely.
- Handle special case of no backups for a tx.
- ha_tests.py: new and modified tests to cover the new functionality.

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1518982 13f79535-47bb-0310-9956-ffa450edef68