Add an 'swh-scheduler task schedule_origins' command to run a task on all origins.

Depends on !82 (closed) Resolves #1536 (closed)

Note: schedule_origin_batches is copied verbatim from swh-indexer, I'm going to open a new diff in the indexer to deduplicate this code.

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Migrated from D1196 (view on Phabricator)