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 
Seems I know you mean ~ 
In HeartbeatThread's run() each loop will using same time 
For example : 
mFixedExecutionIntervalMs is 100ms and mExecutor.heartbeat(); take 10ms ,sleeping 90ms 
when mExecutor.heartbeat(); use 20ms , sleeping 80ms 

 To keep the same time for each heartbeat report