mulit: move payment query code to separate file

We also rename the struct since it is now in its own package
there is no need to prefix it.

rpcserver.go

@@ -7513,7 +7513,7 @@ func (r *rpcServer) ListPayments(ctx context.Context,
 		}
 	}
 
-	query := paymentsdb.PaymentsQuery{
+	query := paymentsdb.Query{
 		IndexOffset:       req.IndexOffset,
 		MaxPayments:       req.MaxPayments,
 		Reversed:          req.Reversed,