p2p/discover: implement v5.1 wire protocol (#21647)

This change implements the Discovery v5.1 wire protocol and
also adds an interactive test suite for this protocol.

p2p/netutil/error.go

@@ -23,3 +23,11 @@ func IsTemporaryError(err error) bool {
 	})
 	return ok && tempErr.Temporary() || isPacketTooBig(err)
 }
+
+// IsTimeout checks whether the given error is a timeout.
+func IsTimeout(err error) bool {
+	timeoutErr, ok := err.(interface {
+		Timeout() bool
+	})
+	return ok && timeoutErr.Timeout()
+}