Implement some tracking of producer positions in the LevelDB store to help the broker to filter out dups.

git-svn-id: https://svn.apache.org/repos/asf/activemq/trunk@1482788 13f79535-47bb-0310-9956-ffa450edef68
Fixes AMQ-4535: Activemq configured with leveldb commit fail when accessed by PutGet from IBM Perf Harness

git-svn-id: https://svn.apache.org/repos/asf/activemq/trunk@1482789 13f79535-47bb-0310-9956-ffa450edef68
https://issues.apache.org/jira/browse/AMQ-4000 - as part of this feature we need to properly send advisories when durable sub unregisters

git-svn-id: https://svn.apache.org/repos/asf/activemq/trunk@1482790 13f79535-47bb-0310-9956-ffa450edef68
Fixing regression in last commit.

git-svn-id: https://svn.apache.org/repos/asf/activemq/trunk@1482794 13f79535-47bb-0310-9956-ffa450edef68
reduce the number of times the sendSyncs map is accessed.

git-svn-id: https://svn.apache.org/repos/asf/activemq/trunk@1482800 13f79535-47bb-0310-9956-ffa450edef68