Access Control: add retrieval methods (#458).

There are methods for retrieving topics/associations based on creator/owner.

Access Control service:
    - Collection<Topic> getTopicsByCreator(String username)
    - Collection<Topic> getTopicsByOwner(String username)
    - Collection<Association> getAssociationsByCreator(String username);
    - Collection<Association> getAssociationsByOwner(String username);

REST API:
    - GET /accesscontrol/creator/{username}/topics
    - GET /accesscontrol/owner/{username}/topics
    - GET /accesscontrol/creator/{username}/assocs
    - GET /accesscontrol/owner/{username}/assocs

Note: these methods rely on the property index recently introduced (4.1.1-SNAPSHOT).
Topics/associations existing in a DM 4.1 installation are not indexed retrospectively.

Close #458.