This will be used to allow http2 servers to register a shutdown function
so that net/http.Server.Shutdown will work when the http2 server is
configured via a manual call to http2.ConfigureServer. Currently, Shutdown
only works when the http2 server is configured automatically by the
net/http package.

Updates #20302
Updates #18471

Change-Id: Ifc2b5f3126126a106b49ea4a7e999279852b9cc9
Reviewed-on: https://go-review.googlesource.com/44003
Run-TryBot: Tom Bergan <tombergan@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>