If a user starts two HTTP requests when no http2 connection is
available, both end up creating new TCP connections, since the
server's protocol (h1 or h2) isn't yet known. Once it turns out that
the server supports h2, one of the connections is useless. Previously
we kept upgrading both TLS connections to h2 (SETTINGS frame exchange,
etc).  Now the unnecessary connections are closed instead, before the
h2 preface/SETTINGS.

Tests in the standard library (where it's easier to test), in the
commit which updates h2_bundle.go with this change.

Updates golang/go#13957

Change-Id: I5af177e6ea755d572b551cc0b0de9da865ef4ae7
Reviewed-on: https://go-review.googlesource.com/18675Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>