lnwire: morph HTLCSettleRequest into UpdateFufillHTLC

This commit modifies the prior HTLCSettleRequest to more closely match
the UpdateFufillHTLC defined within the specification. The only
semantic change is the move from a slice of pre-images (for “multi-sig”
LN) to a single payment preimage.

lnwire/update_fulfill_htlc_test.go

@@ -0,0 +1,30 @@
+package lnwire
+
+import (
+	"bytes"
+	"reflect"
+	"testing"
+)
+
+func TestUpdateFufillHTLCEncodeDecode(t *testing.T) {
+	// First create a new HTLCSR message.
+	settleReq := NewUpdateFufillHTLC(*outpoint1, 23, revHash)
+
+	// Next encode the HTLCSR message into an empty bytes buffer.
+	var b bytes.Buffer
+	if err := settleReq.Encode(&b, 0); err != nil {
+		t.Fatalf("unable to encode UpdateFufillHTLC: %v", err)
+	}
+
+	// Deserialize the encoded SFOP message into a new empty struct.
+	settleReq2 := &UpdateFufillHTLC{}
+	if err := settleReq2.Decode(&b, 0); err != nil {
+		t.Fatalf("unable to decode UpdateFufillHTLC: %v", err)
+	}
+
+	// Assert equality of the two instances.
+	if !reflect.DeepEqual(settleReq, settleReq2) {
+		t.Fatalf("encode/decode error messages don't match %#v vs %#v",
+			settleReq, settleReq2)
+	}
+}