func WriteToSessionUDP(conn *net.UDPConn, b []byte, session *SessionUDP) (int, error) {
	return conn.WriteTo(b, session.raddr)
}
WriteToSessionUDP acts just like net.UDPConn.WriteTo(), but uses a *SessionUDP instead of a net.Addr.
TODO(fastest963): Once go1.10 is released, use WriteMsgUDP.
package dns
import "net"
type SessionUDP struct {
	raddr *net.UDPAddr
}
func (s *SessionUDP) RemoteAddr() net.Addr { return s.raddr }
func ReadFromSessionUDP(conn *net.UDPConn, b []byte) (int, *SessionUDP, error) {
	n, raddr, err := conn.ReadFrom(b)
	if err != nil {
		return n, nil, err
	}
	return n, &SessionUDP{raddr.(*net.UDPAddr)}, err
}