Ability to set a timeframe when certificate renews should occur

[WRMSRwasTaken]

Hi!

I wonder if it'd be possible to add a config option to select a certain timeframe of a day to schedule certificate renews in, for example in the global section and / or in the per-certificate section.
My issue being not all daemons support hot-reloading the TLS certificates and require a full restart (looking at you, OpenVPN), potentially interrupting client traffic as acmed handles the reloading & restarting of the affected services automatically for me.
Thankfully it's just a hand full of daemons that don't support certificate hot-reloading (actually it's really just OpenVPN for me currently).

Maybe this could be called renew_timeframe or something like that to enter something like 03:00-05:00, together with random_early_renew and / or renew_delay then acmed would select a specific point in time matching the time frame to renew the cert.

Thanks!

[jcgruenhage]

An easy workaround for this would be to schedule your OpenVPN restart with cron / systemd instead of from an acmed hook. Whether it makes sense to do this in acmed directly I'm not sure.