(#3220) crontab: allow purging unmanaged resources

This pull was requested earlier this year already. That request was closed because there were issues that couldn't be timely resolved. Apparently, the old request #1605 cannot be re-opened, so here's a new one.

All blockers are removed now. Feature and tests are working.

The solutions may be workaround-ish, and there may be better ways. But I believe that these will work fine.