Restart NAT port timeout measurement when keepalive fails and other fixes

Misc keepalive fixes including:
+ Restart NAT port timeout measurement when keepalive fails. The max interval
  is set to the current keepalive interval.
+ When exception occurs during sending a keepalive, restarts registration.
+ When exception occurs during measurement, retry for a limited times before
  giving up.

Change-Id: I7aa787a5ec7c4c9b4334aa1017371d9049b3520c