Each inode has: image name, parent inode number & pool, size and readonly flag
Snapshots are created by switching image name to a different inode number
while using the older inode as parent.
Previous implementation didn't respect write ordering and could lead
to corrupted data when restarting writes after an OSD outage
Also rework cluster_client queueing logic and add tests for it to verify the correct behaviour
Vitaliy Filippov