celery-monitor: requeue all pending tasks
It can occur, for instance in case of "worker lost", that task events are not sent in the "MONITOR_KEY" list in redis. The proposed solution is to regularly check pending tasks status. Since this is almost the same use, this can replace the old "requeue" behavior of failed sync_task_state() which was based on a "PENDING_KEY" list in redis. So, every 10 min, was 1min before, but 10 min seems more reasonable, check all pending tasks status within the visibility_timeout, since tasks outside this range cannot have state stored in the broker.