Improve category enrolment performance in 2.2 (backport of MDL-34864)

SELECT e.*, ue.userid

              FROM mdl_enrol e

              JOIN mdl_context ctx ON (ctx.instanceid = e.courseid AND ctx.contextlevel = '50')

              JOIN mdl_user_enrolments ue ON (ue.enrolid = e.id)

         LEFT JOIN (SELECT DISTINCT cctx.path, ra.userid

                      FROM mdl_course_categories cc

                      JOIN mdl_context cctx ON (cctx.instanceid = cc.id AND cctx.contextlevel = '40')

                      JOIN mdl_role_assignments ra ON (ra.contextid = cctx.id AND ra.roleid IN ('5','9','11'))

                   ) cat ON (ctx.path LIKE CONCAT(cat.path, '/%') AND cat.userid = ue.userid)

             WHERE e.enrol = 'category' AND cat.userid IS NULL