Additional fix for AMQ-4535: Seems store was getting out of sync /w cursor due to thread unsafe access in the leveldb store.

Switched to a concurrent HashMap to track the transaction since that will be getting accessed by multiple threads.

git-svn-id: https://svn.apache.org/repos/asf/activemq/trunk@1483369 13f79535-47bb-0310-9956-ffa450edef68