Access Control: fix permission cache (#489, #478).

The Access Control module does *not* deliver the permissions along with the topic/association (in the composite value). Instead the Webclient requests the permissions separately via the Access Control service.

To reduce the number of requests the Access Control module maintains its own client-side cache for the permission information, and invalidates it once the user's login status changes.

This fixes the bug of stale permissions (due to browser caching) once the user's login status changes.

For the first time since introduction of timestamps, edit conflict detection, and browser caching DM should be in a good shape now. Please give it a try and report any problems.

Access Control API has 1 new method:
    - Permissions getAssociationPermissions(long assocId)
      RESTful API: GET /accesscontrol/association/{id}
      Cliet-side: dm4c.restc.get_association_permissions(assoc_id)

Close ticket 489.
See ticket 478.