feat: sleep at renewal

[ldez]

Add a sleep between 0 to 8 minutes during the call of the renew command like certbot.

Fixes #1656

[dmke]

I've found some defects :)

[dmke]

@ldez, I have another remark:

@jsha asked for a randomized sleep for non-interactive renewal. The current implementation however requires an extra flag to skip it. Users might be surprised when they run renew manually and don't know of that flag.

Wouldn't it be better, if we (additionally) check (github.com/mattn/isatty).IsTerminal, i.e.

if !isatty.IsTerminal(os.Stdout.Fd()) && !ctx.Bool("no-random-sleep") {
    // ...
}

(N.B.: This is the same approach certbot uses)

isatty is already an indirect dependency, and works under various GOOS, however it might delay moving to Go 1.18: mattn/go-isatty#72

If we don't want to use that, the log.Infof call should also print something like "add --no-random-sleep to skip".

What do you think, shall I open a PR?

[ldez]

@dmke you can open a PR.

For the problem with go1.18 we already use an updated version of golang.org/x/sys.