Simplify worker logic

[imranariffin]

Before #20 it didn't cross my mind to use multiprocessing.Pool to initialize child workers and send tasks to them. Using this would simplify the architecture described in #9. As part of this we should also consider refactoring the worker.py to make it more readable, extensible and configurable.

We should also simplify the subscription logic. I recently learned that we can replace the polling with aioredis.client.PubSub.subscribe (I couldn't the docs for it but it is there in the source code)