Changing the way that altsvc use is indicated

For #443.

The discussion we've had so far on this seems to have converged on the use of ALTSVC frames for indicating what alternative service is in use.  This enacts that change, and adds a congruent use of the Alt-Svc header field for use in contexts other than HTTP/2.

That means that the Service header field can be removed safely.

I've also added some notes regarding tracking of clients.  I think that is sufficient (I'll make this another pull request if you'd prefer it that way).