This is the start of wiring up the HTTP/2 Transport. It is still
disabled in this commit.
This change does two main things:
1) Transport.RegisterProtocol now permits registering "http" or
"https" (they previously paniced), and the semantics of the
registered RoundTripper have been extended to say that the new
sentinel error value (ErrSkipAltProtocol, added in this CL) means
that the Transport's RoundTrip method proceeds as if the alternate
protocol had not been registered. This gives us a place to register
an alternate "https" RoundTripper which gets first dibs on using
HTTP/2 if there's already a cached connection.
2) adds Transport.TLSNextProto, a map keyed by TLS NPN/ALPN protocol
strings, similar in feel to the existing Server.TLSNextProto map.
This map is the glue between the HTTP/1 and HTTP/2 clients, since
we don't know which protocol we're going to speak (and thus which
Transport type to use) until we've already made the TCP connection.