Ceph

Some stuff:
- promoting clones means creating clones without CLONE pg log operations
- promoting head needs to pull and populate the snapset
- need to demote dirty clones older clones first with appropriate snapcontext (for pool snapshots, the demoting osd needs to still be able to provide a snapcontext overriding the pool snapcontext)
- when demoting clones, use clone_overlap to write the correct regions, this causes the backing pool osd to end up with the correct clone overlaps
- whiteout needs to be treated as delete when demoting snapshots
- pool removed_snaps should look at tier_at