acme/autocert: remove failed state entries

This change makes the Manager try creating a certificate
again, after a previously unsuccessful attempt.

The implementation is based on a timer, to prevent hitting
an ACME CA with too high QPS when under a heavy load.
The timer is hardcoded to 1 minute.

Fixes golang/go#17740.
Change-Id: I46a49201cf423be3360633a89209d7b2bccc1d76
Reviewed-on: https://go-review.googlesource.com/41694
Run-TryBot: Alex Vaghin <ddos@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
2 files changed