Escalated from OU Bug 3791, 'Live cron exhausts memory, before "core" jobs including sync_metacourses'

Cron is consistently failing on
   "Removing expired enrolments ...Allowed memory size of 134217728 bytes exhausted..."

The attached patch uses the preferred 'rs_fetch_next_record' call, and only gets required fields, to reduce memory use.
It removes the 'course' table, enrolperiod>0 check and loop, introduced for Bug MDL-10181 (also MDL-8785) - is this really necessary? If so this SQL join could form the basis.

SELECT ra.roleid, ra.userid, ra.contextid
  FROM mdl_course c
  INNER JOIN mdl_context cx ON cx.instanceid = c.id
  INNER JOIN mdl_role_assignments ra ON ra.contextid = cx.id
  WHERE cx.contextlevel = '50'
  AND timeend > 0
  AND timeend < 1189123200;
--AND c.enrolperiod > 0;