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 
It can be a big issue.

Let's say the network gets slow so heart beat gets slow. When this happens then we will start to miss the heart beat interval.  If you monitor time and schedule based off time taken, we make that better.