Cleanup stale servers only if marked DOWN, as the hints from discovery are
to be honored only if server is detected DOWN by agent.

Core is seen due to access of deleted AgentXmppChannel. AgentXmppChannel
was deleted as discovery sent new set of services even though the vrouter
did not detect the server as DOWN. (This happens only during
provisioning as the sytem is loaded and the heartbeats were not
recieved by discovery and it detects the control-node as DOWN)

Change-Id: I5b588baf59d1550d2a359dd5529074a16c873c02
Closes-Bug:1403295