rbd: add header parser for merge-diff

Signed-off-by: Yunchuan Wen <yunchuanwen@ubuntukylin.com>
rbd: add a strategy for merge-diff

We only handle the image-diff with "--stripe-count == 1"

Signed-off-by: Yunchuan Wen <yunchuanwen@ubuntukylin.com>
rbd: add command line for merge-diff

Signed-off-by: Yunchuan Wen <yunchuanwen@ubuntukylin.com>
test: add test case for rbd merge-diff

Signed-off-by: Yunchuan Wen <yunchuanwen@ubuntukylin.com>