[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.
I think that `os.fork()` already handles negative return values by throwing `OSError`, so I think this `else` block is dead code: https://docs.python.org/2/library/os.html#os.fork