swh-loader-core: doesn't properly batch object insertions (for big requests)

The core loader uses a queue system to bundle loading of objects.

This works well when you repeatedly store small amounts of objects, as it allows to bundle up those requests in bigger requests. However, when loading a big number of objects at once (e.g. loading the contents of a big Debian package such as libreoffice), it doesn't split up the insertion in batches.

This needs to be fixed so that loading doesn't overwhelm the database.

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Migrated from T804 (view on Phabricator)