sheep: forbid double locking by single tgtd

Previous tgtd could cause double locking as an ordinal action. But
recent change
(https://github.com/fujita/tgt/commit/744aa51940c10c7986fc3911eb3ea44d6116ed45)
removed this confusing action. So this patch let sheep forbid the
double locking.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
sheep: treat inode object as a sparse object

Sparse object cannot handle erasure coded objects well. But inode
objects are always stored with naive replication. So this patch adds a
change for treating inode objects as a sparse object for reducing disk
space.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>