ThreadPoolExecutor and its children CPUThreadPoolExecutor and IOThreadPoolExecutor

ThreadPoolExecutor and its children CPUThreadPoolExecutor and IOThreadPoolExecutor

Summary:
Spun off from https://phabricator.fb.com/D1534506 as this seemed different enough for a new diff

Similar to previous diff but attempts to reuse a common thread management process between cpu and io bound thread pools. Also sets the stage for other common functionality, e.g. stats, monitoring, timeouts, and so on

Here is some output from the queue benchmark in common/concurrent with both of these pools added (changes to BM not in this diff): https://phabricator.fb.com/P16308560

Test Plan: added a unit test, ran benchmark

Reviewed By: davejwatson@fb.com

Subscribers: fugalh, njormrod, bmatheny

FB internal diff: D1555443

Tasks: 5002392, 5002425