Tasks are simple callables (e.g. lambdas), and can be pushed with 2
priority levels. Pushing a task returns a future.
Futures can be waited on, can return results, and can be cancelled
deterministically. Futures can also not be waited on.
This gives 'hardware concurrency - 1' threads to maximize CPU usage in a
work-stealing workflow.
Reviewed by: vladislavbelov
Refs #5874
Differential Revision: https://code.wildfiregames.com/D3848
This was SVN commit r25656.
