Rudder

If we are unlucky enough to edit a group right when the dynamic group update is running, and saving this specific group, we may have all of our change that is overwritten
In DynGroupUpdater, we:

1. load the group
2. check it's dynamic
3. get its query
4. compute new members
5. save it if they change

Last step should ensure that:
- we don't overwrite query
- if query changed between step 3 and save, rerun the update

Result can be pretty surprising, as group config looks ok, but isn't