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 
Yes maybe . I don't know what are you talking about .
sleeping base make you have a better control But I don't sure it's a good idea . 
recursively schedule? it's just a sequence tasks . 
wish you can make tachyon work better & effective 
PS : fixed-rate-service take a running in fixed time interval