May sometimes be helpful to get more predictable runtime
than just with an age-based limit.
The subquery for the non-keep-threads path is required
since delte_all does not directly accept limit().
Again most of the diff is just adjusting indentation, best
hide whitespace-only changes with git diff -w or similar.
This gives feedback when to stop rerunning limited batches.
Most of the diff is just adjusting indentation; best reviewed
with whitespace-only changes hidden, e.g. `git diff -w`.
This part of pruning can be very expensive and bog down the whole
instance to an unusable sate for a long time. It can thus be desireable
to split it from prune_objects and run it on its own in smaller limited batches.
If the batches are smaller enough and spaced out a bit, it may even be possible
to avoid any downtime. If not, the limit can still help to at least make the
downtime duration somewhat more predictable.
Oneric