QPID-4390: ensure the _messageStore field is populated prior configuring the store to ensure it is set before use by the listener during the asynchronous HA store activation

merged from trunk r1479172


git-svn-id: https://svn.apache.org/repos/asf/qpid/branches/0.22@1479182 13f79535-47bb-0310-9956-ffa450edef68