Implement multiplexed-connections feature

[Barre]

Implement multiplexed connections in order to implement a workaround against issue #95.
This workaround was mentioned by the author of the redis-rs library: redis-rs/redis-rs#325 (comment)

Based on my experience of the last few weeks, the issue didn't occur anymore after using MultiplexedConnection instead of Connection.

[djc]

Hey, thanks. In the issue I asked if you could explain a bit more about why/how the semantics are different.

I don't think doing this as a feature that changes the connection type makes sense. It could just be a separate ManageConnection implementation in bb8-redis, or potentially we should just semver-bump bb8-redis and always use the MultiplexedConnection, since you'll pretty much want these semantics for pooling anyway.