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

Merged edu-services/trunk: r117605, r118469, r118507



git-svn-id: https://source.sakaiproject.org/svn/edu-services/branches/edu-services-1.2.x@118509 66ffb92e-73f9-0310-93c1-f5514f145a0a