Ceph

What's the purpose of this? User versions are "user" only in the sense that they're the versions we expose to them as operations. They still follow system rules and are basically built off of the pg version, though!

My recollection is that we preserve them when moving objects in/out of the cache tier. I assume we want them to also be preserved if you do a full pool export/import, which means the rados import tool needs to be able to set it via librados.