Saturating semaphore

Saturating semaphore

Summary:
SaturatingSemaphore is a flag that allows:
- multiple concurrent posters
- multiple concurrent waiters
- idempotent posting
- non-destructive waiting
- blocking and spinning
- pre-block spin time control

```
/// SaturatingSemaphore is a flag that allows concurrent posting by
/// multiple posters and concurrent non-destructive waiting by
/// multiple waiters.
///
/// A SaturatingSemaphore allows one or more waiter threads to check,
/// spin, or block, indefinitely or with timeout, for a flag to be set
/// by one or more poster threads. By setting the flag, posters
/// announce to waiters (that may be already waiting or will check
/// the flag in the future) that some condition is true. Posts to an
/// already set flag are idempotent.
```

Reviewed By: djwatson

Differential Revision: D6379704

fbshipit-source-id: 59aed76caa2d159639e75425a778a9c63f18f375