- strict policy on sql transaction (commit all or nothing for each task)
- make it work with on_monitor_start (used to synchronize task states when
  monitor starts in case of missing events)
- Use serializable() in "spawn" task so it force celery to use the json task
  serializer instead of pickle
- Don't always update task_name when creating subtasks outside of celerytask
  (eg. by using start_async_task), use a fixed identifier "<unknown>" as task
  name instead and only update these.