global executors with weak_ptr semantics

global executors with weak_ptr semantics

Summary:
unfortunately, can't use atomics now that a weak_ptr is stored instead of a raw ptr, hence the additional singleton locks
might want to make an overload of via() that takes shared_ptr to integrate more easily? or change via() to store a shared_ptr (and make it non-owning if a raw ptr is passed)

Test Plan: unit

Reviewed By: davejwatson@fb.com

Subscribers: jsedgwick, trunkagent, fugalh, folly-diffs@

FB internal diff: D1764359

Tasks: 5002442

Signature: t1:1764359:1420845340:349ea88091d7ca4ee386b54aec599647341fadd4