Update ClusterStatsResponse

[gmarz]

https://www.elastic.co/guide/en/elasticsearch/reference/master/breaking_50_rest_api_changes.html#_removed_literal_mem_literal_section_from_literal__cluster_stats_literal_response

and

https://www.elastic.co/guide/en/elasticsearch/reference/master/breaking_50_rest_api_changes.html#_revised_node_roles_aggregate_returned_by_literal__cluster_stats_literal