systemd unit dbus subscriptions should use unit events.

Basically this TODO needs to be implemented:

func (s *SubscriptionSet) Subscribe() (<-chan map[string]*UnitStatus, <-chan error) {
	// TODO: Make fully evented by using systemd 209 with properties changed values
	return s.conn.SubscribeUnitsCustom(time.Second, 0,
		mismatchUnitStatus,
		func(unit string) bool { return s.filter(unit) },
	)
}

can't accurately monitor units being restarted without calling SubscribeUnitsCustom with a very short delay. which wastes a bunch of cpu.

Start at SubscriptionSet.Subscribe and the current SubscribeUnitsCustom call. Read the systemd 209 unit-events and PropertiesChanged behavior to understand how unit status changes should be delivered without short polling. Done means restarts are monitored accurately through unit events while avoiding the current CPU-heavy delay-based subscription.