channeldb: eliminate extra copy in QueryPayments

In this commit, we eliminate an extraneous copy in the `QueryPayments`
method. Before this commit, we would copy each payment from the initial
FetchPayments call into a new slice. However, pointers to payments are
return from `FetchPayments`, so we can just maintain that same reference
rather than copying again when we want to limit our response.

rpcserver.go

@@ -5191,7 +5191,7 @@ func (r *rpcServer) ListPayments(ctx context.Context,
 	for _, payment := range paymentsQuerySlice.Payments {
 		payment := payment
 
-		rpcPayment, err := r.routerBackend.MarshallPayment(&payment)
+		rpcPayment, err := r.routerBackend.MarshallPayment(payment)
 		if err != nil {
 			return nil, err
 		}