BUG 789: Stats Mgr to skip equality check on flow stats

BUG 789: Stats Mgr to skip equality check on flow stats

- Statistics Manager runs an equality check on the list of flow statistics
  received from the protocol plugin with its cached ones to decide whether
  to update or not, in order to avoid a cluster cache update.
  In presence of flows on the switch, the flow updated statistics are always
  different becasue of the milli and nano secs duration fields they contain

Change-Id: I8290457c1d0a43294e2449cb16c14d6c1e36481e
Signed-off-by: Alessandro Boch <aboch@cisco.com>
Signed-off-by: Surekha Bejgam <sbejgam@cisco.com>