Add journal client implementation which updates content archiver db with new contents
This leverages the base class client (swh-journal!235 (closed)) to do thy bidding.
Note that this is a draft and some implementation needs improvment.
I mainly target the archiver storage functions (existing one + that new one in this revision) which should be improved/refactored/merged + one of the archiver director which is already able to cope with missing entries... (#569 (closed))
Related T494
Migrated from D181 (view on Phabricator)