routing: new newLogClosure to delay spew evaluation until log print time

In this commit, we update the generateSphinxPacket to use newLogClosure
to delay the spew evaluation until log print time. Before this commit,
even if we weren't on the trace logging level, the spew call would
always be evaluated.

routing/router.go

@@ -1458,7 +1458,10 @@ func generateSphinxPacket(route *Route, paymentHash []byte) ([]byte,
 	hopPayloads := route.ToHopPayloads()
 
 	log.Tracef("Constructed per-hop payloads for payment_hash=%x: %v",
-		paymentHash[:], spew.Sdump(hopPayloads))
+		paymentHash[:], newLogClosure(func() string {
+			return spew.Sdump(hopPayloads)
+		}),
+	)
 
 	sessionKey, err := btcec.NewPrivateKey(btcec.S256())
 	if err != nil {
@@ -1467,8 +1470,9 @@ func generateSphinxPacket(route *Route, paymentHash []byte) ([]byte,
 
 	// Next generate the onion routing packet which allows us to perform
 	// privacy preserving source routing across the network.
-	sphinxPacket, err := sphinx.NewOnionPacket(nodes, sessionKey,
-		hopPayloads, paymentHash)
+	sphinxPacket, err := sphinx.NewOnionPacket(
+		nodes, sessionKey, hopPayloads, paymentHash,
+	)
 	if err != nil {
 		return nil, nil, err
 	}