[Sheepdog] support object recovery

[Piavlo]

Hi

Thanks for the explanations.

some more questions regarding current recovery implementation:
With current recovery what is supposed to happen then one of the  nodes
goes down with --copies=N redundancy:
1) Are the vm blocks get replicated to other nodes so that there are
still N copies of each vm block in active cluster?
If yes does this happen in the background, it the recovery replication
triggered automatically?
2)Same questions regarding the vdi meta directories.

> Yes, vdi directory is like metadata, which we calld `super object' before.
> Currently, the redundancy of vdi directory is same as objects redundancy.
> Perhaps, we should change it because vdi directory is more important than
> data objects.
>   
Since this vdi metadata overhead is very small , it is probably
reasonable to store the vdi metadata for all vm images in the sheepdog
cluster
on each storage node. This would not require any metadata recovery if
some node goes offline.
> How to decide the nodes to store vdi directory is depends on consistent hashing,
> so the stored nodes may change if you add or remove nodes.
>