Upgrade to containerd v1.4

[impl]

This is a tracking issue for upgrading the containerd library dependency for the containerd shim to the v1.4 branch when possible. We currently have to maintain some compatibility code in the shim that would be unnecessary with a newer containerd minor version.

[fvoznika]

We need to wait for Kubernetes 1.17, which uses containerd 1.3, to reach end of life before we can start using 1.4. Otherwise we loose the ability to issue security patches to 1.17 if needed. This link has the schedule for release deprecations. 1.17 end of life is set to 2022-01.

Note: containerd is backwards compatible with older shims, e.g. containerd 1.3 works with shims built with 1.2. However, shims are not guaranteed to be backward compatible with older containerd, e.g. shim built with 1.3 interfacing with containerd 1.2.