Improve the N tag policy to retain exactly N tags

[cd1989]

For the moment, if some tags share a same image manifests, they will be bind together, we can't delete some of them and to retain others.

If t1, t2, t3, t4 refer to the same image and we want to delete tags t1, t2. Then we can:

• Hold manifest of the image
• Delete t1 (so all 4 tags will be deleted)
• Push t3, t4 back by pushing the manifest, since blobs already in registry, no need to push blobs.

[cd1989]

Done.