[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.
As an experiment, I added the line

```scala
simpleWorkers(socket).destroy()
```

and, as expected, this results in a `java.lang.ProcessBuilder cannot be cast to java.lang.Process` error.

The compiler should have prevented this, so I think we've found a compiler bug.