[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.
It looks like `simpleWorkers` is declared as a map of `Process` but here you're storing a `ProcessBuilder`; IntelliJ displays this as an error, but it still seems to compile.  Any idea what's going on here?