Function record_obsolete 

fn record_obsolete(
    obsolete_entry_ids: &Arc<Mutex<HashMap<RegionId, EntryId>>>,
    catalog: &RwLock<ObjectCatalog>,
    region_id: RegionId,
    entry_id: EntryId,
)

Records the obsolete watermark of region_id, which never moves down, for reads and in the catalog for the reclaim boundary.