APLO-245: The LevelDB store does not seem to get cleaned/compacted

We now periodically monitor the size of the index and compact if it has grown too large relative to the number of entries we have stored in it.

git-svn-id: https://svn.apache.org/repos/asf/activemq/activemq-apollo/trunk@1439549 13f79535-47bb-0310-9956-ffa450edef68