* Fixing contents of `omap_val` - it doesn't contain an oid at all (but the way the values were packed hid the problem really well!)
* Maths for computing `obj_id` for a file key have to use `|`, not `+`, otherwise negative values break, and negative values are used in the snapshot info block.
* Naming a value which is the reference to the volume superblock for the snapshot.
* Marking some other unknown stuff which is clearly obj references as `ref_obj`.