Modify  TachyonWorker Heartbeat sleeping to ScheduledExecutorService 

TachyonWorker's heartbeat thread is sleeping several milliseconds . 
JDK API ScheduledExecutorService's scheduleAtFixedRate method running a task after initial delay and subsequently with the given period. It's more fit for the heartbeat task .
And a TODO : split heartbeat thread into another class.
thanks 
sleep several ms for send a heartbeat message maybe work fine but seems not a good idea . while-true loop will do a lot work which could be avoid. using scheduled service could running task after specified ms . I think it's same with sleeping base interval and scheduled service .