sheep: garbage collect needless VIDs

Current sheepdog never recycles VIDs. But it will cause problems
e.g. VID space exhaustion, too much garbage inode objects.

Keeping deleted inode objects is required because living inodes
(snapshots or clones) can point objects of the deleted inodes. So if
every member of VDI family is deleted, it is safe to remove deleted
inode objects.

Cc: Saeki Masaki <saeki.masaki@po.ntts.co.jp>
Cc: Yuka Kawasaki <kawasaki.yuka@po.ntts.co.jp>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>