The change should reduce resource quota consumed by the client overall.

Instead of sending HEAD to an ACME resource URL to get a new nonce,
the Client will now try to fetch it from the Directory URL first
and only then from the ACME resource URL if the former fails.

This builds up on an abandoned https://golang.org/cl/34623,
only this time with a fallback to the original behaviour.

Change-Id: I6e75c0e524c4bc751f3a651b290c0ac2493e0628
Reviewed-on: https://go-review.googlesource.com/c/162057
Run-TryBot: Alex Vaghin <ddos@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>