This makes ConfigureServer initialize the http2 Server's IdleTimeout
from the http1 Server configuration, using the same rules as
IdleTimeout in Go 1.8: first use IdleTimeout if specified, else fall
back to the old ReadTimeout.

Updates golang/go#14204

Change-Id: I4dee971f8416ef0cbf99335a199c46355f9ab09d
Reviewed-on: https://go-review.googlesource.com/32230
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tom Bergan <tombergan@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>