When creating a self signature based on an existing signature, make sure the new signature has a newer signature creation time

Consider VaildKeyAmalgamation::set_validity_period_as_of. It uses the current binding signature as a template. But if the current binding signature's creation time is the same as 'now', then we can't be sure that the "new" signature will be used. Instead, we should make sure that the new signature's signature creation time is at least the template's creation time + 1 second.