Make bitbucket origins ingestion concurrent

The current execution is running slowly for now. It's a simple loop over the mapping file (one line is one origin) to trigger the ingestion per origin.

Make it go faster using the actual loader_oneshot.

For this, we need to iterate over the mapping file and send tasks swh.loader.mercurial.tasks.LoadMercurial with the proper parameters.

That also have the following pros:

• no need to deploy another special worker

• have the systemctl logs pushed to elk, which make it browsable more easily (at least for staff) [1]

• [1] http://kibana0.internal.softwareheritage.org:5601/goto/23e431d304850e45e602752e849a3f28

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Migrated from T3455 (view on Phabricator)