Add the PubSub Channel/Receiver back in as an extension module

[seandstewart]

It doesn't have the MPSC module (aioredis.pubsub.Receiver), which I use in a lot of places.

I can see now how removing those abstractions could be a pretty large blocker for some. Could you create an issue describing how you use the Channel/Reciever abstraction? I can look at adding it back in, perhaps as a contrib or ext module to denote that it is an extension over the default implementation.

Originally posted by @seandstewart in #891 (comment)

[Andrew-Chen-Wang]

@gjcarneiro how did you use to use it? Are you thinking about subclassing PubSub class? https://aioredis.readthedocs.io/en/latest/getting-started/#pubsub-mode