Support for HTTP/1.1 Upgrade ala Section 3.2 of the new soon-to-be RFC

[jsipprell]

The following PR adds support for HTTP/1.1 upgrading ala Section 3.2.

The public interface for this is quite similar to ConfigureServer:

var h2srv *http.Server
var conf *http2.Server
http2.UpgradeServer(h2srv, conf).ListenAndServe()

Internally this is performed by cloning h2serv before calling http2.ConfigureServer() and hooking its handler. When a request is detected which meets the upgrade requirements (Connection, Upgrade and HTTP2-Settings headers), the settings payload is decoded, the HTTP/1.1 portion of the upgrade is finalized, the connection is flushed and then hijacked. Finally, the hijacked connection is passed off to http2's handleConn.

In order to facilitate injection of a hijacked net.Conn into http2's state machine some minor changes to server.go were made:

1. handleConn() was refactored slightly by adding a new func: newConn() which takes an optional pre-state []Setting slice and an upgrade callback function.
2. A new member was added to the serverConn struct: a single-use upgrade channel which returns a func() that will be called once all settings frames have been ack'd. This callback is used to mock the original HTTP/1.1 request as a header frame payload on the half-closed stream id 1 (as per the spec). A final call to processHeaderBlockFragment() triggers downstream application request processing as per usual.

NOTE: Because the spec includes the dubious requirement that any request entity bodies must be read in-full before the protocol upgrade can begin, any attempts to transmit a request body are detected and will result in a silent failure to upgrade and a fallback to HTTP/1.1 handling.

This has been tested using Curl 7.42.0-DEV built against nghttp2/0.7.12-DEV as well as nghttp2 itself. Unit tests are provided which require docker & gohttp2/curl (via http2_test.go).

[bradfitz]

This repo just moved into the official Go repo.

See https://github.com/bradfitz/http2/blob/master/README for the move details.

File new bugs at: https://github.com/golang/go/issues/new?title=x/net/http2:+

Closing this PR as we're now using Gerrit for code review. Please file a bug or send the review to Gerrit if this is still relevant. Sorry for how long this languished here.