Implement a global task manager using a pool of worker threads

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