Ceph » rbd

- when we open an image, open the parent image too.
- make reads fall through to parent
- guard writes beyond parent size with object existence check
- on failure, attempt copyup and retry

test plan:
- create child, read child, compare to parent
- create child, modify child, compare to parent
- resize child (up, down, both), verify we get the correct fall-thru behavior
- modify librbd_fsx to periodically create a child and shift new work to it