channeldb: split cancelHTLCs logic in the UpdateInvoice method

Previous to this commit we were able to call `UpdateInvoice` with an
updated that added and cancelled htlcs at the same time. The function
returned an error if there was overlapping between the two htlc set.
However, that behavior was not used in the LND code itself.

Eventually we want to split this method in multiple ones, among them one
for canceling htlcs and another one for adding them. For that reason,
this behavior is not supported anymore.

channeldb/invoice_test.go

@@ -479,6 +479,7 @@ func TestInvoiceCancelSingleHtlc(t *testing.T) {
 		invoice *invpkg.Invoice) (*invpkg.InvoiceUpdateDesc, error) {
 
 		return &invpkg.InvoiceUpdateDesc{
+			UpdateType: invpkg.CancelHTLCsUpdate,
 			CancelHtlcs: map[models.CircuitKey]struct{}{
 				key: {},
 			},
@@ -554,6 +555,7 @@ func TestInvoiceCancelSingleHtlcAMP(t *testing.T) {
 		invoice *invpkg.Invoice) (*invpkg.InvoiceUpdateDesc, error) {
 
 		return &invpkg.InvoiceUpdateDesc{
+			UpdateType: invpkg.CancelHTLCsUpdate,
 			CancelHtlcs: map[models.CircuitKey]struct{}{
 				{HtlcID: 0}: {},
 			},
@@ -616,6 +618,7 @@ func TestInvoiceCancelSingleHtlcAMP(t *testing.T) {
 			error) {
 
 			return &invpkg.InvoiceUpdateDesc{
+				UpdateType: invpkg.CancelHTLCsUpdate,
 				CancelHtlcs: map[models.CircuitKey]struct{}{
 					{HtlcID: 1}: {},
 				},
@@ -643,6 +646,7 @@ func TestInvoiceCancelSingleHtlcAMP(t *testing.T) {
 		invoice *invpkg.Invoice) (*invpkg.InvoiceUpdateDesc, error) {
 
 		return &invpkg.InvoiceUpdateDesc{
+			UpdateType: invpkg.CancelHTLCsUpdate,
 			CancelHtlcs: map[models.CircuitKey]struct{}{
 				{HtlcID: 2}: {},
 			},