func (ca *Agent) Subscribe(subscription DeltaChan) {
	ca.mut.Lock()
	defer ca.mut.Unlock()
	ca.subscriptions = append(ca.subscriptions, subscription)
}
func (ca *Agent) Subscribe(subscription DeltaChan) {
	ca.mut.Lock()
	defer ca.mut.Unlock()
	ca.subscriptions = append(ca.subscriptions, subscription)
}
// Subscribe registers the channel for messages on config reload.
// The caller can expect a copy of the previous and current config
// to be sent down the subscribed channel when a new configuration
// is loaded.