Implement redis broadcaster using aioredis

[tizz98]

The existing aioredis implementations were incomplete (#8 and #14). This PR adds support for aioredis and properly handles subscribing and unsubscribing from multiple channels using the aioredis.pubsub.Receiver class which allows for "multiple producer, single consumer".

This PR also fixes a bug with the MemoryBackend where the backend wasn't usable outside of tests. The problem was the asyncio.Queue was created on __init__ instead of on connect so the wrong event loop was being used.

Additionally, a new test is added to verify connected with a redis password works. SSL is also supported but cannot be easily tested with an untrusted CA because of how aioredis handles the ssl parameter. It expects it to be true or false.

[willingc]

@tomchristie and other maintainers, Thanks for writing this project and for all of the work in the Encode ecosystem. Awesome packages. As this is currently an alpha project, what are the plans, if any, to create an updated release with some of these PRs merged? Thoughts?

[AntonOfTheWoods]

@tizz98 , from the looks of your repo, it appears you are thinking about creating a broadcaster-notable, given @tomchristie hasn't had the time to answer any of the issues posted here in the last few months. Is there an ETA on that?

[tizz98]

@AntonOfTheWoods you can use it now, it's publised on pypi