Participants page use of get_users_roles exhausts memory with many site-wide role assignments

This needs a regression test of the Particiants page (user/index.php). So, perhpas re-run the related MDLQA tests. A good specific test that the code change did not cause a regression is: Logged in as admin, find a course with more than 20 enrolled users. Go to the participants page (Course administration -> Users -> Enrolled user) for that course. Verify that the table displays without errors, and that, in particular, the values in the Roles column are right. Go to the second page of results, and check that too. For one of the users in the table, assign them a System role (e.g. Manager) using Site administration -> Users -> Permissions -> Assign system roles. Re-load the particpants table, and verify that the new role shows up.