support updating objectClasses for a User

This PR does a few things:

- moves existing deactivatedUid and new objectClass meta-data to a common implementation meta-data HashMap.
- implementation meta-data is not observed for object equality
- updates objectClasses based on current state of User object
- always updates objectClass on object update
Given that this meta data is, as you say, an 'internal implementation detail', what about protecting it by some means so that only internal code has access to it?