QPID-5800: [Java Broker] Refactor BDB message store implementation to separate message and config store implementations.

* Message store implementations can now be used in isolation, which is useful when the user is using a JSON VirtualHostNode with
  a BDB virtual host.

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1603849 13f79535-47bb-0310-9956-ffa450edef68