Core: refactor storage layer, pt.2 (#391).

Does not yet compile.

API Changes:

- Class AssociationModel
    - Add getOtherRoleModel(long id) convenience method.
- Class ResultSet
    - Add a default constructor to construct an empty result set.
    - Add addAll() method to combine result sets.

BREAKING CHANGES:

- Interface AssociationRole
    - replace getAssociationId() by getPlayerId()
- Interface TopicRole
    - replace getTopicId() by getPlayerId()

See ticket 391.