Gerrit contains following changes :

Gerrit contains following changes :
1) Fix for bug 284. Added functionality that listen to data store *remove* changes and
cleanup the relevant statistics from operational data store.
2) Added functionality that listen to config data store *create* changes for Flow,Group,
Meter & Queue and send statistics request to switch to get respective stats.
3) Added functionality to periodically remove stale stats from operational data store.
Clean up thread invokes after every two cycle of stats collection.
4) Removed unnecessary local caching.

Change-Id: Ibee3c73905ce872302c4f54ce5b7b53c0657ee51
Signed-off-by: Anil Vishnoi <avishnoi@in.ibm.com>