bpo-35493: Use Process.sentinel instead of sleeping for polling worker status in multiprocessing.Pool (#11488)

* bpo-35493: Use Process.sentinel instead of sleeping for polling worker status in multiprocessing.Pool

* Use self-pipe pattern to avoid polling for changes

* Refactor some variable names and add comments

* Restore timeout and poll

* Use reader object only on wait()

* Recompute worker sentinels every time

* Remove timeout and use change notifier

* Refactor some methods to be overloaded by the ThreadPool, document the cache class and fix typos