Schedule recurring fetches of origin metadata

We decided in #1739 (closed) to fetch origin metadata at the same time we load artefacts.

However, we should also fetch metadata from time to time, for repositories which do not get any update (because listers don't schedule loads when they detect there is no update); especially for repositories with no load since we introduced metadata fetchers.

(cc @zack )

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Migrated from T4252 (view on Phabricator)