This is a better fix that https://golang.org/cl/43455. Instead of
creating a separate goroutine to wait for the global shutdown channel,
we reuse the new serverMsgCh, which was added in a prior CL.

We also use the new net/http.Server.RegisterOnShutdown method to
register a shutdown callback for each http2.Server.

Updates golang/go#20302
Updates golang/go#18471

Change-Id: Icf29d5e4f65b3779d1fb4ea92924e4fb6bdadb2a
Reviewed-on: https://go-review.googlesource.com/43230
Run-TryBot: Tom Bergan <tombergan@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>