The Server distinguishes "new" vs "idle" connections. A TCP connection
from which no bytes have yet been written is "new". A connection that
has previously served a request and is in "keep-alive" state while
waiting for a second or further request is "idle".

The graceful Server.Shutdown historically only shut down "idle"
connections, with the assumption that a "new" connection was about to
read its request and would then shut down on its own afterwards.

But apparently some clients spin up connections and don't end up using
them, so we have something that's "new" to us, but browsers or other
clients are treating as "idle" to them.

This CL tweaks our heuristic to treat a StateNew connection as
StateIdle if it's been stuck in StateNew for over 5 seconds.

Fixes #22682

Change-Id: I01ba59a6ab67755ca5ab567041b1f54aa7b7da6f
Reviewed-on: https://go-review.googlesource.com/121419
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>