channeldb+invoices: move hold invoice settle logic into registry

This commit is a continuation of the centralization of invoice state
transition logic in the invoice registry.

channeldb/invoice_test.go

@@ -692,8 +692,9 @@ func getUpdateInvoice(amt lnwire.MilliSatoshi) InvoiceUpdateCallback {
 		}
 
 		update := &InvoiceUpdateDesc{
-			State:   ContractSettled,
-			AmtPaid: amt,
+			Preimage: invoice.Terms.PaymentPreimage,
+			State:    ContractSettled,
+			AmtPaid:  amt,
 		}
 
 		return update, nil