QPID-5800: [Java Broker] Refactor BDBConfigurationStore, move methods/fields associated with only BDBMessageStore to the inner class.

Addresses review comment from Alex Rudyy.

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1602437 13f79535-47bb-0310-9956-ffa450edef68