exit ThreadPoolExecutor immediately

[woooee at gmail.com]

Doug Hellmann has what looks like a similar example using a poison pill (2nd example at) https://pymotw.com/2/multiprocessing/communication.html#multiprocessing-queues  Note that join() allows the processes to finish so it you don't care then don't "join()" them.  You can also terminate a multiprocessing thread https://pymotw.com/2/multiprocessing/basics.html#terminating-processes