[CA-125859], [SCTX-1588]: Fixed the slow AD logon for users with large group memberships.
- group names are not cached when the other user details are cached (in the constructor), but only when the UserDetails.GroupMembershipNames property is first used.
- also optimized the code for getting user details.
Merge pull request #11 from MihaelaStoica/master

[CA-125859], [SCTX-1588]: Fixed the slow AD logon for users with large group memberships