Summary:
We need the ability to dynamically change the priority of an executor when it's executing work. Specifically, we want to de-prioritize a query's work units while it is being executed if it has taken up too much resources.
Pass a callback function to ExecutorWithPriority which sets the priority upon every add.

Reviewed By: andriigrynenko

Differential Revision: D18344806

fbshipit-source-id: c0c59d3982637703e6e7c3793930b9d9fadce6fe