Merge for 2.9.1: SAK-22978 - Improve gradebook performance for external items

Merged sam/trunk: r117607, r118471



git-svn-id: https://source.sakaiproject.org/svn/sam/branches/samigo-2.9.x@118511 66ffb92e-73f9-0310-93c1-f5514f145a0a