A graveyard exists as a place to park objects that should be deleted when they are no longer in
use. How objects enter and leave the graveyard is up to the caller to decide. The intention is
that at mount time, any objects in the graveyard will get removed. Each object store has a
directory like object that contains a list of the objects within that store that are part of the
graveyard. A single instance of this Graveyard struct manages all stores.