Summary:
[Folly] A semaphore type for testing scenarios which need a basic portable semaphore. Supports FIFO and LIFO wake policies.

May be used when testing other, more sophisticated, semaphore types.

Reviewed By: strager

Differential Revision: D15843438

fbshipit-source-id: 9136e8e19ea79fce4191e69795d5fe848a482f03