invoices: treat replayed HTLCs beforehand.

We make sure that HTLCs which have already been decided upon
are resolved before before allowing the external interceptor to
potentially cancel them back. This makes the implementation for
the external HTLC interceptor more streamlined.

invoices/invoices.go

@@ -765,6 +765,9 @@ type InvoiceStateUpdateDesc struct {
 
 // InvoiceUpdateCallback is a callback used in the db transaction to update the
 // invoice.
+// TODO(ziggie): Add the option of additional return values to the callback
+// for example the resolution which is currently assigned via an outer scope
+// variable.
 type InvoiceUpdateCallback = func(invoice *Invoice) (*InvoiceUpdateDesc, error)
 
 // ValidateInvoice assures the invoice passes the checks for all the relevant