Update registrar to use start_watching_system() API
|Reported by:||anevilyak||Owned by:||bonefish|
Currently the registrar does a polling sanity check in order to detect teams which have gone away without unregistering properly (i.e. in the case where a team is killed/crashes rather than quitting normally). This could be eliminated by instead using the new start_watching_system() call to monitor for team creation/destruction.