Throw exception if no database can be retrieved from the pool (instead of returning null), and retry up to the configured amount of times (JM-1336, JM-1337). Reviewed by Daniel.

git-svn-id: http://svn.igniterealtime.org/svn/repos/openfire/trunk@10268 b35dd754-fafc-0310-a699-88a17e54d16e