Wrap multistream-select streams under a Negotiated struct

[tomaka]

In preparation for #659, let's wrap all the multistream-select streams that succeed under a Negotiated struct.

Right now this is quite useless, but in the future the Negotiated would turn into something like:

pub enum Negotiated<T> {
    WaitingForHandshakeFinished(T),
    Finished(T),
}

This PR only adds the Negotiated so that we can discuss this part alone.

[twittner]

I am not sure how this will work out eventually, but as an informative newtype Negotiated looks good to me.