QPID-5720: HA exception raised by the store for durable transactions

Running qpid-txtest against a HA cluster was raising this exception:
async_dequeue() failed: jexception 0x0b02 wmgr::dequeue_check() threw JERR_MAP_LOCKED: Record ID locked by a pending transaction.

This is actually a test bug. In a cluster a transaction commit takes longer to
complete because of co-ordinating with backups, the test was not waiting for the
completion of commit command before proceeding. Adding a sync() in the test
solves the problem.  Note this test uses the old obsolete C++ API. Applications
written using the supported messaging API are not affected by this. This can be
verified using the qpid-send and qpid-receive clients with the --tx option
(there is already an automated test for this in ha_tests.py.)

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1589520 13f79535-47bb-0310-9956-ffa450edef68