acme: add function to check rate limits

This change exposes a function to extract rate limit duration
from a client error using Retry-After response header.
Author: David Calavera <david.calavera@gmail.com>.

Fixes golang/go#19304.
Change-Id: Iec9cfab398b84c6f216b95d3265ffad1ce2f29a7
Reviewed-on: https://go-review.googlesource.com/37463
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
2 files changed