Storage fix: update assoc index on delete (#389).

BREAKING CHANGES

Core API: rename methods
{{{
    RelatedTopic.getAssociation()                         ->  getRelatingAssociation()
    RelatedTopicModel.getAssociationModel()               ->  getRelatingAssociation()
    RelatedAssociationModel.getRelatingAssociationModel() ->  getRelatingAssociation()
}}}

See ticket 389.