Process hang problem in client process for JobServer

[wynot12]

#1191 is to introduce JobServer and its main part is client code.

This client communicates with Driver through REEF's client message channel, and with other client processes (they submit a command to the main client process) via Java socket.

The problem is that when closing the JobServer including its driver and client, client process does not quit automatically after the end of main() method.

@JunhoeKim has resolved the problem by manually calling `System.exit(0)'. We may explore it later.