Core API: remove "clientState" parameter (#496).

BREAKING CHANGE

The `clientState` parameter is dropped from the Core API's and standard plugin service's get/retrieve/delete methods.

DeepaMehtaService
    - getTopic(long id, boolean fetchComposite)
    - getTopic(String key, SimpleValue value, boolean fetchComposite)
    - getTopics(String key, SimpleValue value, boolean fetchComposite)
    - getTopics(String topicTypeUri, boolean fetchComposite, int maxResultSize)
    - searchTopics(String searchTerm, String fieldUri)
    - deleteTopic(long topicId)

    - getAssociation(long assocId, boolean fetchComposite)
    - getAssociation(String assocTypeUri, long topic1Id, long topic2Id,
                     String roleTypeUri1, String roleTypeUri2, boolean fetchComposite)
    - getAssociationBetweenTopicAndAssociation(String assocTypeUri, long topicId, long assocId,
                     String topicRoleTypeUri, String assocRoleTypeUri, boolean fetchComposite)
    - deleteAssociation(long assocId)

    - getTopicType(String topicTypeUri)
    - getAssociationType(String assocTypeUri)
    - getAllTopicTypes()
    - getAllAssociationTypes()

DeepaMehtaObject
    - getRelatedTopic(String assocTypeUri, String myRoleTypeUri, String othersRoleTypeUri, String othersTopicTypeUri,
                     boolean fetchComposite, boolean fetchRelatingComposite)
    - getRelatedTopics(String assocTypeUri, int maxResultSize)
    - getRelatedTopics(String assocTypeUri, String myRoleTypeUri, String othersRoleTypeUri, String othersTopicTypeUri,
                     boolean fetchComposite, boolean fetchRelatingComposite, int maxResultSize)
    - getRelatedTopics(List assocTypeUris, String myRoleTypeUri, String othersRoleTypeUri, String othersTopicTypeUri,
                     boolean fetchComposite, boolean fetchRelatingComposite, int maxResultSize)

TopicmapsService
    - Topicmap getTopicmap(long topicmapId)

GeomapsService
    - Topic getGeoTopic(long topicId)

=> If your plugin uses these methods it must be adapted.

See #496.