Add a journal client base class to process messages
This is a first draft to implement a base client from our swh-journal.
This can be used for example to trigger the creation of new content in the archiver db.
Related #424 (closed) Related T494
Outside of that scope, i see some possible refactoring between the client class (a consumer) and the publisher (which is also a consumer). Also between the publisher and the swh.storage.listener (the listener is a publisher).
Migrated from D180 (view on Phabricator)