`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successful heal and a no-op since the outdated disks were offline.
In the algorithm to check if an object requires healing, in addition to
checking if all disks have xl.json present we should check if all parts
of the object are present and have valid blake2b checksums.
Also fixed a minor compilation error in heal-objects-list.go.
