Summary:
Add a simple scheduler which operates in the following way.
There are two type of tasks: main, background.
The main task will run on the main thread whilst the background task will run on a background thread.
The main task will be executing at a fixed internal whereas the background task will get queued on demand but can effectively consume what was produced by the main task at its own rate.
Reviewed By: LukeDefeo
Differential Revision: D38975283
fbshipit-source-id: 0633385d2938705a16f5fc75a28cad067e4a8e55
1b2f875cc6