Improve membership listing speed by caching groups associated with Memberships

PrivilegeHelper.canViewMemberships(GrouperSession s, Collection c) iterates across the Collection of Memberships and calls getGroup/getStem each time. Very often the Group/Stem will be the same for each Membership. Currently the API repetitively instantiates the same object - a relatively slow operation, expecially at Bristol where we have a number of custom types and attributes.

Using Ehcache should greatly improve performance.