Consolidate resolver package with containerd.

[Random-Liu]

Today, to support registry mirror in cri, we copied containerd docker resolver package https://github.com/containerd/containerd/blob/master/remotes/docker/resolver.go, and maintained it at https://github.com/containerd/cri/blob/master/pkg/containerd/resolver/resolver.go.

This introduced a lot of code duplication. And we have to keep porting fix to our own resolver package, e.g. #722.

We should:

• Either move the registry mirror logic into containerd (may need a more comprehensive proposal).
• Or at least make most code in docker/resolver a library that we can reuse in cri.

@abhi

[Random-Liu]

I don't think we can finish this issue this release. Move to next.