query: 2.1.0 - Refined input JSON (adapted to general format for stats)

* Adapted the app to the new type-mapping `stats` for the incomming keys.
* Refined the class `Key`, that defines the JSON for the incomming key.
** Renamed attribute `user` to `channel`.
** Added attribute `type` of type `String`.
* Refined the class `Entry`, that defines the JSON of an entry in the
  ranking, that is defined in the class `Ranking`.
** Renamed attribute `word` to `key`.
* The `QueryStreamProcessor` filters the incomming messages by the field
  `type` of the `Key`: all messages are dropped, that are not of type
  `COUNTER`.