Add journal client to fetch metadata from origins not visited in a while
A future MR will add a CLI endpoint to run it. (I added it now)
I recommend reviewing this MR commit-by-commit, as the first two are mostly noise.
This does the bulk of the work for swh/meta#4394 (closed)
Tests will fail as mypy depends on swh-loader-core!451 (merged)
Edited by vlorentz