The trick with this one is to use `futures::Either` everywhere where we may wrap something that implements any of the `futures` traits. This includes the output of `EitherFuture` itself. We also need to implement `StreamMuxer` on `future::Either` because `StreamMuxer`s may be the the `Output` of `InboundUpgrade`.
We can completely replace `EitherFuture2` with `EitherFuture`. `EitherFuture` itself cannot be removed for now because the `Future` implementation on `future::Either` forces both `Future`s to evaluate to the same type.
