MXS-1929: Take ResultSet into use

Replaced the previous RESULTSET with the new implementation. As the new
ResultSet doesn't have a JSON streaming capability, the MaxInfo JSON
interface has been removed. This should not be a big problem as the REST
API offers the same information in a more secure and structured way.

server/core/internal/modules.hh

@@ -21,4 +21,4 @@
  *
  * @return A Result set
  */
-ResultSet *moduleGetList();
+std::unique_ptr<ResultSet> moduleGetList();