Access Control: API is polymorph (#472).

BREAKING CHANGE

The getters and setters for creator/owner/acl do not distinguih between topics and associations but take a generic DeepaMehtaObject.

    String getCreator(DeepaMehtaObject object);
    void setCreator(DeepaMehtaObject object, String username);

    String getOwner(DeepaMehtaObject object);
    void setOwner(DeepaMehtaObject object, String username);

    AccessControlList getACL(DeepaMehtaObject object);
    void setACL(DeepaMehtaObject object, AccessControlList acl);

See #472.