As a further optimization we could also limit the lower bound of the _modified
range. The revision GC does not need to check documents with a _deletedOnce
again if they were not modified after the last successful GC run. If they
didn't change and were considered existing during the last run, then they
must still exist in the current GC run. To make this work, we'd need to
track the last successful revision GC run.
Lowest last validated _modified can be possibly saved in settings collection and reused for next run