staging: Deploy swh.loader.mercurial v2.1.0

Here goes the plan:

• storage1.staging: Upgrade swh.storage to v0.35 (mind the migration step)
• Make the staging check end-to-end for mercurial loader fail (as in production) by duplicating extids (rough sql magic inside).
• workers: Upgrade swh.model and swh.loader.mercurial, restart workers
• The failing staging check end-to-end should go green again
• Clean up: Drop the duplicated extids

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Migrated from T3447 (view on Phabricator)