`disksUnavailable` healStatus constant indicates that a given object
needs healing but one or more of disks requiring heal are offline. This
can be used by admin heal API consumers to distinguish between a
successful heal and a no-op since the outdated disks were offline.
This is written so that to simplify our handler code
and provide a way to only update metadata instead of
the data when source and destination in CopyObject
request are same.
Fixes#3316