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

Merged from assignment/trunk: r117608, r118472



git-svn-id: https://source.sakaiproject.org/svn/assignment/branches/assignment-2.9.x@118512 66ffb92e-73f9-0310-93c1-f5514f145a0a