channeldb: split cancel and add htlc updates

Previously the cancel and add actions were combined in a single map.
Nil values implictly signaled cancel actions. This wasn't very obvious.
Furthermore this split prepares for processing the adds and cancels
separately, which is more efficient if there are already two maps.

invoices/update.go

@@ -130,7 +130,9 @@ func updateInvoice(ctx *invoiceUpdateCtx, inv *channeldb.Invoice) (
 		},
 	}
 
-	update := channeldb.InvoiceUpdateDesc{Htlcs: newHtlcs}
+	update := channeldb.InvoiceUpdateDesc{
+		AddHtlcs: newHtlcs,
+	}
 
 	// Don't update invoice state if we are accepting a duplicate payment.
 	// We do accept or settle the HTLC.