Start journal client to populate the origin_visit_stats model
This is only a function not plugged to anything yet. Remains the cli part to actually be able to use it.
Questions inline in the code as comment.
Related to #2967 (closed)
Test Plan
tox (failing until swh.model > 0.9 [1] is released)
Migrated from D4852 (view on Phabricator)