Do not schedule all bulk import subtasks at once

[rkorytkowski]

Celery does not handle well a chain of multiple chords with groups that have a lot of tasks. See celery/celery#5286 From my testing with 8 chained chords and groups with ~200 tasks in total it starts to fail for us.

I tried to limit the number of tasks, but the more chords you have in a chain, the lower the limit is. It is undesired for us to schedule less that 200 tasks for an import with e.g. 60 000 resources.

My solution is to schedule just one chord at a time and have a callback task that schedules another chord.