_worker_handler has two issues:

* It polls the worker status every status every 100 ms: I created bpo-35493 to investigate how to avoid that
* After close() or terminate() has been called, it loops until self._cache is empty. I would like to use result.wait(), but a result never completes after terminate(): I created bpo-35478 to see if tasks can be unblocked in that case (to ensure that result.wait() completes after terminate().