Add a dask distributed backend to coffea.compute

[nsmith-]

A draft implementation is at #1470 (comment)

@pfackeldey follow up here with any further comments. A question for you: for the background thread, do you think it makes sense to share it across backends since its somewhat common to just need something to do work while the user thread continues to do whatever it does? (probably most of the time it blocks on task.result())

[pfackeldey]

I think that's a good idea, do you think it's possible to hide this in some BackendBase, such that all concrete implementations do not have to think about the background thread at all?

[nsmith-]

It might be good, also as a way to handle any common configuration arguments for all backends. So far I only really have one: how to handle running tasks when the context manager exits (i.e. if a user does not wait the tasks inside the context manager): the choices are either to block until all tasks are complete or to cancel all tasks.