refactor the code related to xtcp (#3449)

pkg/util/net/udp.go

@@ -256,3 +256,11 @@ func (l *UDPListener) Close() error {
 func (l *UDPListener) Addr() net.Addr {
 	return l.addr
 }
+
+// ConnectedUDPConn is a wrapper for net.UDPConn which converts WriteTo syscalls
+// to Write syscalls that are 4 times faster on some OS'es. This should only be
+// used for connections that were produced by a net.Dial* call.
+type ConnectedUDPConn struct{ *net.UDPConn }
+
+// WriteTo redirects all writes to the Write syscall, which is 4 times faster.
+func (c *ConnectedUDPConn) WriteTo(b []byte, addr net.Addr) (int, error) { return c.Write(b) }