bench: use 20 hops for the benchmark

Turns out in practice, the 27 hop limit is actually _too large_, so we
fall back to the old 20 hop limit for now. We'll want to circle back and
emperically derive the _true_ hop limit.

Author: Roasbeef

bench_test.go

@@ -21,7 +21,7 @@ func BenchmarkPathPacketConstruction(b *testing.B) {
 		route        PaymentPath
 	)
 
-	for i := 0; i < NumMaxHops; i++ {
+	for i := 0; i < 20; i++ {
 		privKey, err := btcec.NewPrivateKey()
 		if err != nil {
 			b.Fatalf("unable to generate key: %v", privKey)