Keep indexer table in sync with new contents

Symmetrically to the archiver module, the indexer module needs to update its data references (content sha1 for now) when new contents are added.

Leveraging the journal infrastructure to have a journal client that inserts new references for indexation in the index queue (orchestrator_all) sounds the way forward.

Related #494 (closed)

Migrated from T874 (view on Phabricator)

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