Investigate stream-based execution model as a replacement for current scheduler/celery/task based approach

This task is dedicated to start a discussion on the opportunity to replace part/all our (celery) task-based architecture for handling background processing of the archive.

First come in mind replacing the task-based indexers by (kafka) journal consumer ones. But we should consider the whole scheduler/celery/task model as well.

Also, we should not stick to kafka for now. Let's have in mind all the reasonable alternatives, with pros/cons of each one of them.

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Migrated from T2063 (view on Phabricator)