fix issue tachyon-200  If master gets restarted, workers never reregister

Remove the throws exception logic  from worker heartbeat thread when master get down .  The master's reponsibilty is store meta data and coordinate workers. I think when master down. client can continue read from work if it has already connect to the worker. but write should throws Exception.
by the way when master use Zookeeper as HA solution. this kind of case is rarely happened.
@xbkaishui Here is a left issue. Thanks.