Summary:
Adds a new TrackThreadId template argument to SharedMutexImpl that when enabled, expands SharedMutex from 4 to 8 bytes (still with
4 byte alignment), and uses the extra space to record the thread ID
of a distinguished owning thread (upgrade or exclusive lock holder).
This dramatically simplifies debugging in some scenarios.  It adds enough
information that we could enforce that unlock happens on the same thread
as lock, but this diff doesn't actually add such a check.

A new TrackedSharedMutex class is added for SharedMutex with lock owner thread id tracking enabled. fb_localtime is the only user for now to reduce the risk.

Differential Revision: D24731513

fbshipit-source-id: 6b41fb05498842224feb088b1332794281b501a9