add periodically updated cached message stats

[andreaskoepf]

• add new endpoints:
  GET /api/v1/stats/cached
  GET /api/v1/stats/cached/{name}
  POST /api/v1/cached/update

Initially available stats:

    "human_messages_by_lang"
    "human_messages_by_role"
    "message_trees_by_state"
    "message_trees_states_by_lang"
    "users_accepted_tos"

• default update period is set to 1h, update is not executed during server startup but after first period...