Access Control: namespace properties (#491, #458).

DB property keys are namespaced:
    - "creator" -> "dm4.accesscontrol.creator"
    - "owner"   -> "dm4.accesscontrol.owner"
    - "acl"     -> "dm4.accesscontrol.acl"

A migration that transforms the DM 4.1 property keys is included.
Note: while migrating the "creator" and "owner" values are indexed. So the new user-based retrieval methods can be used also when updating from DM 4.1 (#458).

See #491.
See #458.