loader-metadata: restart the journal client when it reaches EOF

We expect this journal client to run continuously, refreshing old metadata when necessary; to do so, we need to set the journal client to reset itself on EOF.

Ref. swh/infra/sysadm-environment#5289

approved this merge request

I had stuck the commit on the next-version branch to test it out, and it got merged as e2754d53

closed