Many celery-task-meta keys stored in redis

[xavierfav]

I noticed that there were a lot of celery-task-meta keys stored in Redis in the production server (around 500 today).
It does not seem very problematic for now, but I think it should not be the case.
Maybe it is related to this Celery issue.
In this case, we should try to upgrade Celery (our version is 4.1.1).