[Enhancement] Make transport pluggable

[bmerry]

I'm working on a new library for mocking redis, for which I intend to provide both redis-py and aioredis integrations. The base of the library works by using a socketpair, one end of which behaves like a redis server and the other end replaces the client's normal TCP or Unix socket.

For aioredis, I've got something working but I'm having to duplicate a fair bit of code and deal with internals (more than for redis-py), because the logic to create a (reader, writer) pair is mixed in with unrelated logic about connection classes, timeout validation etc in create_connection. It would be nice if create_connection could take a factory that could turn an address into a (reader, writer) pair, similar to the way connection_cls is a factory (and obviously also pass it through from higher levels like create_redis).

I'm happy to do the work and make a PR, but wanted to file this as an issue first in case anyone has already done something like this or has thoughts on how to architect it. I've never used aioredis before today so feedback will certainly help me avoid making a mess of it.