Implement Children method for containerd image store which makes the
`ancestor` filter work for `docker ps`. Checking if image is a children
of other image is implemented by comparing their rootfs diffids because
containerd image store doesn't have a concept of image parentship like
the graphdriver store. The child is expected to have more layers than
the parent and should start with all parent layers.
Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>
