Fix MB-834

"ContentRemovalTimeDifference" configuration value is used to determine
the time to wait before removing message content from Cassandra after
an ack is received from a subscriber.

Fix https://wso2.org/jira/browse/MB-834
Fix for MB-828
Merge pull request #121 from sperera/master

Fix for MB-828
Fixing some of the warnings, made perf related changes, removed deliveredButNotAckedMessages hash map since that map was not used operationally and only put and remove were performed on it and not get
Changes
Merge pull request #122 from pamod/mqtt_dev

Perf Related Enhancements Made
Merge pull request #120 from a5anka/content_removal

Fix MB-834
Record DLC queue added for standalone mode.
Merge pull request #123 from akalankapagoda/DLCStandalone

Record DLC queue added for standalone mode.