rbd-mirror: support cloning an image from a non-primary snapshot
In the case of OpenStack Glance, this capability would allow the creation of volumes from a DRed non-primary copy of an image. Proper support would most likely require implementing a deletion "recycling bin" to fix the corner cases with parent/child links and propagating of deletions (i.e. the clone of the non-primary parent image might not be mirrored and the primary parent image would not know about the child on the non-primary side. the primary image could be deleted but that deletion would not properly propagate since the non-primary image would have a child).