rbd: copy-on-read for clones, read entire object from parent

Signed-off-by: Min Chen <minchen@ubuntukylin.com>
rbd: copy-on-read for clones, write entire object into child by cls_rbd copyup

Signed-off-by: Min Chen <minchen@ubuntukylin.com>
rbd: copy-on-read for clones, add an configure option for copy-on-read

Signed-off-by: Min Chen <minchen@ubuntukylin.com>