QPID-5801: [Java Broker] BDB: Simply restore algorithm to efficiently calculate utilised disk space.

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1601346 13f79535-47bb-0310-9956-ffa450edef68
QPID-5801: [Java Broker] BDB: Cache the sequence handler used for message sequence number

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1601351 13f79535-47bb-0310-9956-ffa450edef68