server.go
func (s *Server) Ping(node *net.UDPAddr, callback func(krpc.Msg, error)) error {
	s.mu.Lock()
	defer s.mu.Unlock()
	return s.ping(node, callback)
}
func (s *Server) Ping(node *net.UDPAddr, callback func(krpc.Msg, error)) error {
	s.mu.Lock()
	defer s.mu.Unlock()
	return s.ping(node, callback)
}
// Sends a ping query to the address given.