[SPARK-4694]Fix HiveThriftServer2  cann't stop In Yarn HA mode.

HiveThriftServer2 can not exit automactic when changing the standy resource manager in Yarn HA mode.
The scheduler backend was aware of the AM had been exited so it call sc.stop to exit the driver process but there was a user thread(HiveThriftServer2 ) which was still alive and cause this problem.
To fix it, make a demo thread to detect the sparkContext is null or not.If the sc is stopped, call the ThriftServer.stop to stop the user thread.