[sheepdog] read/write during recovery

[Dietmar Maurer]

>> My naïve patch looks like this (can be optimized further):

>IIUC, your patch does not handle write requests because write
>journaling is not implemented yet, yes?  I think it is not easy to
>implement journaling across nodes.  Do you have any ideas to implement
>it simply?

The idea is to simply discard those write request. We can do that, because 
there is at least one node which has data locally, and that node applies all 
writes (we sync data from that node later).

The only problem is a race condition, because gateway node does not 
update all copies at the same time. But we can solve that by other means.

- Dietmar