lnwallet: restore unsigned acked remote updates

This commit updates the channel state machine to
persistently store remote updates that we have received a
signature for, but that we haven't yet included in a commit
signature of our own.

Previously those updates were only stored in memory and
dropped across restarts. This lead to the production of
an invalid signature and channel force closure. The remote
party expects us to include those updates.

channeldb/channel_test.go

@@ -524,10 +524,34 @@ func TestChannelStateTransition(t *testing.T) {
 	// First update the local node's broadcastable state and also add a
 	// CommitDiff remote node's as well in order to simulate a proper state
 	// transition.
-	if err := channel.UpdateCommitment(&commitment); err != nil {
+	unsignedAckedUpdates := []LogUpdate{
+		{
+			LogIndex: 2,
+			UpdateMsg: &lnwire.UpdateAddHTLC{
+				ChanID: lnwire.ChannelID{1, 2, 3},
+			},
+		},
+	}
+
+	err = channel.UpdateCommitment(&commitment, unsignedAckedUpdates)
+	if err != nil {
 		t.Fatalf("unable to update commitment: %v", err)
 	}
 
+	// Assert that update is correctly written to the database.
+	dbUnsignedAckedUpdates, err := channel.UnsignedAckedUpdates()
+	if err != nil {
+		t.Fatalf("unable to fetch dangling remote updates: %v", err)
+	}
+	if len(dbUnsignedAckedUpdates) != 1 {
+		t.Fatalf("unexpected number of dangling remote updates")
+	}
+	if !reflect.DeepEqual(
+		dbUnsignedAckedUpdates[0], unsignedAckedUpdates[0],
+	) {
+		t.Fatalf("unexpected update")
+	}
+
 	// The balances, new update, the HTLCs and the changes to the fake
 	// commitment transaction along with the modified signature should all
 	// have been updated.