SAK-23514 Use a copy on write collection to prevent threading issues.

This switches to using a copy on write sorted set to store the expanded folders. This prevents the runaway threads that occur when a collection is modified whilst it is being iterated over. I'm not sure why the set was originally sorted.

git-svn-id: https://source.sakaiproject.org/svn/content/trunk@134418 66ffb92e-73f9-0310-93c1-f5514f145a0a