[sheepdog] read/write during recovery

[Dietmar Maurer]

> >  1. There are two node A and B (redundancy level is 2), and each node
> >     has one object.
> >  2. Node C joins Sheepdog, and new placement of the object becomes
> >     node B and C.
> >  3. A VM writes data to the object, and node B completes the request
> >     but node C rejects it since recovery is not started.
> >  4. Node B crashes before node C gets the updated data from node B,
> >     and then the written data will be lost even though only one node
> >     fails.  In addtion, the VM can reads the old object after the
> >     failure, which breaks the block device semantics.

But that is also a problem with the current code. for example when 
node B is the gateway node?

- Dietmar