Restrict permissions to set max_session_mem_used (and maybe more) to SUPER or another suitable admin privilege

There are a number of variables which a low-privileged and/or unknowing user shouldn't be able to change, as it can cause a variety of problems. Since there are already session variables restricted to certain privileges, apparently the necessary mechanism exists within the server, but it would make sense to extend the list (maybe significantly).

max_session_mem_used starts the list. More can be added to the task.