Add a backfiller cli command
This command allowd to backfill a kafka journal from an existing Postgresql provenance storage.
The command will run a given number of workers in parallel. The state of the backfilling process is saved in a leveldb store, so interrupting and restarting a backfilling process is possible, with limitations: it won't work properly if the range generation is modified.
Migrated from D8964 (view on Phabricator)