QueuedImmediateExecutor tweaks

QueuedImmediateExecutor tweaks

Summary:
[Folly] `QueuedImmediateExecutor` tweaks.

* Add a leaky meyers singleton instance.
* Make the thread-local queue an instance variable. Callers which want the singleton thread-local queue can use the singleton executor instance instance, for the same effect.
* Simplify the body of `add`, and perform the thread-local lookup only once per invocation.

Reviewed By: djwatson

Differential Revision: D6399067

fbshipit-source-id: 03904885a70c4b943141bd83868414d27232fd6a