Create a general interface to implement parallel tasks easily and apply it to...

Create a general interface to implement parallel tasks easily and apply it to run EliminateUnreachableBlocks in parallel.

Summary:
Each time we run some work in parallel over the list of functions in bolt, we manage a thread pool, task scheduling and perform some work to manage the granularity of the tasks based on the type of the work we do.

In this task, I am creating an interface where all those details are abstracted out, the user provides the function that will run on each  function, and some policy parameters that setup the scheduling and granularity configurations.

This will make it easier to implement parallel tasks, and eliminate redundant coding efforts.

(cherry picked from FBD16116077)