#2 detaching a volume with snapshot

[YAMAMOTO Takashi]

(on the snapshot branch,)
when detaching a volume with snapshots, vastsky detaches snapshots first, and then detaches the snapshot-origin volume.
it can be a problem if the snapshot-origin volume is actively accessed on the head server, because
- modifications to the snapshot-origin with its snapshots detached will not involves cow and effectively corrupts snapshots.
- the volume will be left in the halfly-detached but still accesible state after the detach attempt failed because of the reference to the snapshot-origin.