http2/h2c: support direct hand off of h2c-upgrade connections

The initial request on an h2c-upgraded connection is written as an
HTTP/1 request, with the response sent as an HTTP/2 stream.

The h2c package handled this request by constructing a sequence of
bytes representing an HTTP/2 stream containing the initial request,
prepending those bytes to the remainder of the connection, and
presenting that to the HTTP/2 server as if no upgrade had happened.
This translation did not handle request bodies. Handling request
bodies under this model would be difficult, since it would require
also translating the HTTP/2 flow control.

Rewrite the h2c upgrade to explicitly hand off the request to the
HTTP/2 server instead.

Fixes golang/go#52882.

Change-Id: I26e0f12e2b1c8b48fd36ba47baea076424983553
Reviewed-on: https://go-review.googlesource.com/c/net/+/407454
Reviewed-by: Brad Fitzpatrick <[email protected]>
Run-TryBot: Damien Neil <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: neild