Fixes#23017
When deleting segments while our footprint is over the limit,
mainly when recycling/deleting segments after replay (recover
boot) we can cause two deletion passes to be running at the same
time. This is because delete is triggered by either
a.) replay release
b.) timer check (explicit)
c.) timer initiated flush callback
where the last one is in fact not even waited for. If we are
considering many files for delete/recycle, we can, due to task
switch, end up considering segments ok to keep, in parallel,
even though one of them should be deleted. The end result
will be us keeping one more segment than should be allowed.
Now, eventually, this should be released, once we do deletion
again, but this can take a while.
Solution is to simply ensure we serialize deletion. This might
cause some delay in processing cycles for recycle, but in
practice, this should never happen when we are in fact under
pressure.
Small unit test included.
4ed81e05bf