recv_trailers should return a concrete type

[yoshuawuyts]

Request::recv_trailers and Response::recv_trailers should return non-clone channels, matching their Sender counterparts. I'm currently integrating this, and because we have an anonymous future calling it from a poll context means we need to box it, which isn't great.

[yoshuawuyts]

Also there's no decent way to handle the errors sent at that point, unless we only allow for io errors to be passed along. I think it's better to acknowledge operations regarding trailers should never fail and go from there.