[Sheepdog] support object recovery

[MORITA Kazutaka]

Hi all,

I've updated the next branch.

git://sheepdog.git.sourceforge.net/gitroot/sheepdog/sheepdog next

Object recovery is partially supported.
If multiple nodes are down sequentially, object recovery wouldn't work.
But otherwise, lost object should be recovered correctly.

Thanks.

-- 
MORITA, Kazutaka