make updates more efficient regardless either dandiset with zarrs or not

ATM we have two cron jobs `tools/backups2datalad-update-cron` (ran often) and `tools/backups2datalad-update-cron-108` (ran long) since 108 contained zarrs and their backup is much more involved (see e.g. #363 ) than of regular files. But now other dandisets also start to contain zarrs. We need to figure out a workflow to perform updates in such a fashion that we do not need some custom separation across dandisets.

I think overall we should start using some proper job system to orchestrate updates. May be even a full blown celery with that flower to monitor the status? Then workflow could be

- given a dandiset with changed time stamp, and no ongoing already job to update it, schedule an `update_dandiset` job which
- for all zarr assets check if they exist, not being uploaded, and up-to-date (based on date).
- if any missing - schedule a job to have zarr created/updated
- if any out of date - schedule a job to have zarr updated
- we might need a "registry" of jobs since can't query celery for ongoing/planned jobs so we skip dandiset if any job is still running
- in any of above cases, skip updating the dandiset in this round
- if no zarrs - or all zarrs found up to date [*], proceed with update of the dandiset as we do now

[*] alert -- race condition, unless we collect specific commits for each zarr so we update them to those and would be fine even if zarr is being modified