[SPARK-1740] [PySpark] kill the python worker

Kill only the python worker related to cancelled tasks.

The daemon will start a background thread to monitor all the opened sockets for all workers. If the socket is closed by JVM, this thread will kill the worker.

When an task is cancelled, the socket to worker will be closed, then the worker will be killed by deamon.
If an Exception raised, n is not defined.

This threads will sleep 0.1 after every poll, the overhead will be low, it should not effect another process reading the socket.