image list: speed up

Listing images has shown increasing performance penalties with an
increasing number of images.  Unless `--all` is specified, Podman
will filter intermediate images.  Determining intermediate images
has been done by finding (and comparing!) parent images which is
expensive.  We had to query the storage many times which turned it
into a bottleneck.

Instead, create a layer tree and assign one or more images to nodes that
match the images' top layer.  Determining the children of an image is
now exponentially faster as we already know the child images from the
layer graph and the images using the same top layer, which may also be
considered child images based on their history.

On my system with 510 images, a rootful image list drops from 6 secs
down to 0.3 secs.

Also use the tree to compute parent nodes, and to filter intermediate
images for pruning.

Signed-off-by: Valentin Rothberg <rothberg@redhat.com>

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