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

Merged gradebook/trunk: r117606, r118470, r118508



git-svn-id: https://source.sakaiproject.org/svn/gradebook/branches/sakai-2.9.x@118510 66ffb92e-73f9-0310-93c1-f5514f145a0a