[sheepdog] read/write during recovery

[Dietmar Maurer]

> If node membership change is detected, object recovery (object
> replacement) start on
> all nodes.  If we delay starting recovery and reject I/O requests until it, all
> nodes cannot accept I/Os during the time and it causes EIO to VMs.

I fully understand that. but you can delay recovering of 'unused' object (only
handle prio_oids, and do other oids later)?