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 
ScheduledExecutorService provides scheduleWithFixedDelay method to take a fixed delay between two times heartbeat . Maybe take same behavior ?? 

But I think it's NOT SO IMPORTANT . Because the difference is so small . Do you think so ?