Use a thread pool

[adamghill]

The current implementation is very straight-forward: put functions in a dequeue and then pop them off and start a thread when request_finished is signalled.

However, instead of a dequeue, it could use a ThreadPoolExecutor.

Potential benefits:

• prevents resource exhaustion if a ton of functions are scheduled
• re-uses threads so they don't need to be created/destroyed for every scheduled function

Not sure if the code complexity is worth it, though.