invoices: add resolution result to htlcResolution

This commit adds the resolution result obtained
while updating an invoice in the registry to
htlcResolution. The field can be used by calling
functions to determine the outcome of the
update and act appropriately.

invoices/update.go

@@ -49,6 +49,10 @@ const (
 	// ResultSettled is returned when we settle an invoice.
 	ResultSettled
 
+	// ResultCanceled is returned when we cancel an invoice and its associated
+	// htlcs.
+	ResultCanceled
+
 	// ResultInvoiceNotOpen is returned when a mpp invoice is not open.
 	ResultInvoiceNotOpen
 
@@ -59,6 +63,10 @@ const (
 	// ResultMppInProgress is returned when we are busy receiving a mpp payment.
 	ResultMppInProgress
 
+	// ResultMppTimeout is returned when an invoice paid with multiple partial
+	// payments times out before it is fully paid.
+	ResultMppTimeout
+
 	// ResultAddressMismatch is returned when the payment address for a mpp
 	// invoice does not match.
 	ResultAddressMismatch