func (router *Router) OnGossip(update []byte) (GossipData, error) {
	_, newUpdate, err := router.applyTopologyUpdate(update)
	if err != nil || len(newUpdate) == 0 {
		return nil, err
	}
	return &topologyGossipData{peers: router.Peers, update: newUpdate}, nil
}
func (router *Router) OnGossip(update []byte) (GossipData, error) {
	_, newUpdate, err := router.applyTopologyUpdate(update)
	if err != nil || len(newUpdate) == 0 {
		return nil, err
	}
	return &topologyGossipData{peers: router.Peers, update: newUpdate}, nil
}
// OnGossip receives broadcasts of TopologyGossipData.
// It returns an "improved" version of the received update.
// See peers.ApplyUpdate.