Trigger subscription from discovery client for faster convergence

o Trigger subscribe from discovery client instead of relying on the discovery client library to resend
  subscribe on ttl expiry. This is added for faster convergence.
o Discovery Server takes atleast 15secs to mark the services down, during with the discovery client(vrouter)
  can attempt connecting to older control-node's advertised by discovery server.
o Xmpp StateMachine support to trigger the connection attempt failure information to the Xmpp Clients
  to take action i.e trigger subscribe to Discovery Server.
o Add a jitter while attempting reconnects.
o Cleanup stale Xmpp and DNS servers after getting updates from discovery, this will avoid indefinite
  reconnects to Servers that are down.

Change-Id: I6f204c8ee269c2c8a31757dc10aa90bcdf3723eb
Closes-Bug: 1395099
Merge "Trigger subscription from discovery client for faster convergence" into R2.0