DERBY-6733: Implement an MBean for monitoring caches

Add an MBean for the statement cache.

Fix the values returned from getAllocatedEntries() and
getUsedEntries(). The first patch got them mixed up.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1624974 13f79535-47bb-0310-9956-ffa450edef68