Each time the Caddyfile reloads and Caddy is restarted,
caddytls.NewConfig starts a goroutine for cleaning the
certificate storage. This goroutine ranges over a time.Ticker
channel; although Stop is called on this ticker, Stop does
not close the underlying channel so the goroutine never exits.

This change adds an additional channel that is listened to
in the certificate cleaning goroutine so it can exit
on restarts.