metadata-loader journal client is way too slow

https://grafana.softwareheritage.org/goto/U5PGQYy4z?orgId=1 shows it is barely matching the rate of new origins, and has 9 years of ETA; while we expected it to go through all origins every 3-4 months.

(This looks like swh/infra/sysadm-environment#4612 but I doubt they are related, as they don't run on the same machines, and I don't see any Kafka-related error in Sentry for the metadata-loader)