QPID-4944: HA fix failure of test_failover_send_receive on slow machines.

At the end of this test, receivers read remaining messages on queues.
Previously the test limited the max depth at 1024, but this was
too high on some slower machines and caused a timeout. The test now
limits the max depth to 50, which seems to work on a range of machines.
There's no impact on the effectiveness of the test for failover.

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1523094 13f79535-47bb-0310-9956-ffa450edef68