QPID-4944: HA re-enable test_failover_send_receive

Appears to have been fixed at this point on trunk, not clear which checkins are
responsible. Test ran for 48 hours with no failures.

Other minor changes:
- Enable test_failover_send_receive
- Increase heartbeat interval.
- Reduce capacity of senders in failover test to be more aggressive.
- Use HaBrokerTest as test base

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1522711 13f79535-47bb-0310-9956-ffa450edef68