lnwire: add observance of optional data loss fields to ChannelReestablish

In this commit, we add support within lnwire for the optional dataloss
fields in ChannelReestablish. With these fields, it’s possible to:
verify that the remote node really knows of the state of our prior
local commitment, and also that they’ve sent us the current commitment
point for their current state.

In the event of dataloss, it’s possible for the party which lost data
to claim their commitment output in the remote party’s commitment if
they broadcast their current commitment transaction.

lnwire/lnwire_test.go

@@ -451,6 +451,31 @@ func TestLightningWireProtocol(t *testing.T) {
 				return
 			}
 
+			v[0] = reflect.ValueOf(req)
+		},
+		MsgChannelReestablish: func(v []reflect.Value, r *rand.Rand) {
+			req := ChannelReestablish{
+				NextLocalCommitHeight:  uint64(r.Int63()),
+				RemoteCommitTailHeight: uint64(r.Int63()),
+			}
+
+			// With a 50/50 probability, we'll include the
+			// additional fields so we can test our ability to
+			// properly parse, and write out the optional fields.
+			if r.Int()%2 == 0 {
+				_, err := r.Read(req.LastRemoteCommitSecret[:])
+				if err != nil {
+					t.Fatalf("unable to read commit secret: %v", err)
+					return
+				}
+
+				req.LocalUnrevokedCommitPoint, err = randPubKey()
+				if err != nil {
+					t.Fatalf("unable to generate key: %v", err)
+					return
+				}
+			}
+
 			v[0] = reflect.ValueOf(req)
 		},
 	}