L2switch - Enhacement of Hosts expiry feature .. need solution independant of host timeouts

Current host expiry feature makes sure that host are expired once the hard/idle time outs expire. There can be more generic ways to solve the problem.

one possible way of implementation:- Keep checking the flows on the switch, if there are no corresponding flows for a particular host on the switch it(host) can be removed from both the local / md-sal data store and can be termed as expired.