Introduced new generalized JSON-format for `top10`

* Added an attribute `type`, that denotes the type of the statistics.
* Introduces a common wording for the other attributes:
** Renamed `user` to `channel`.
** Renamed `word` to `key` (in the `key`/`counter`-pairing in the `Entry`).
* Adapted all services accordingly:
** `counter` generates statistics of the type `COUNTER`.
** `popular` generates statistics of the type `POPULAR`.
** `top10` generates `Ranking`s for _all_ types of statistics and hands
   the `type` through to its consumers.
** `query` is only intrested in rankings of the type `COUNTER`.