ch-image: rename images in opaque storage

[j-ogas]

It would be nice to change the name of images in opaque storage. Things to consider: 1) architecture hierarchy, and 2) changing the name of the corresponding image manifest and fat manifest?

[j-ogas]

With the addition of the build cache, #1255, we can revisit this.

#1282 proposed simply renaming the image. This implied, at least to me, that the build cache would simply rename the branch. This would make all images referencing the previous image reference via FROM miss. This is still an option.

However the build cache also gives us the ability to implement a tag-like feature. The following.

$ ch-image list
foo
$ ch-image tag foo bar
[...]

The above would keep the worktree, foo, and add another worktree, bar, that is based on foo. This allows future references to foo to still hit.