Core: revise association def model (#393).

BREAKING CHANGES

These assoc def fields are dropped from serialized form:
    - assoc_type_uri        # the same as assoc's "type_uri"
    - whole_type_uri        # can be get from assoc's roles
    - part_type_uri         # can be get from assoc's roles
    - whole_role_type_uri   # fixed: "dm4.core.whole"
    - part_role_type_uri    # fixed: "dm4.core.part"

AssociationModel:
    - AssociationModel(long id, String typeUri) constructor is dropped
    - AssociationModel(long id, String typeUri, RoleModel roleModel1, RoleModel roleModel2) constructor
      has additional "uri" argument (String) behind "id"
    - setRoleModel1() and setRoleModel2() methods are dropped

See ticket 393.