Count authors from revisions and releases

The default counters are using only the journal's message keys for perfomance reason and to be completely agnostic regarding the king of messages on a topic.

But sometimes, detailed information stored in the messages needs to be counted, like in this case, the authors.

It will need a less generic journal client in charge of deserializing the messages and count other specific fields.

---

Migrated from T3251 (view on Phabricator)