Consider migrating the worker backend to `concurrent.futures`

[phargogh]

Taskgraph currently relies on a system of multiprocessing.Event objects and a multiprocessing.Pool to manage processes. Since taskgraph was released, concurrent.futures has emerged as a way to solve most of the same kinds of problems that Task objects were intended to solve. It would be worth taking a closer look at concurrent.futures and seeing what would make sense as a worker backend going forward.