[sheepdog] read/write during recovery

[Dietmar Maurer]

> Due to the fact that we are not easily to get the agreement(one of most
> difficult aspect in distributed system), we don't actually remove the object
> that is already migrated.
> 
> We have a command collie cluster cleanup to manually remove these unused
> objects. Also, restarting sheep will trigger an internal cleanup operation of
> that sheep too.

So the previous  suggestion would be a way to solve that problem cleanly.