We let dockershim implement the kubelet's internal (CRI) API as an
intermediary step before transitioning fully to communicate using gRPC.
Now that kubelet has been communicating to the runtime over gRPC for
multiple releases, we can safely retire the extra interface in
dockershim.
Implement CRI stats for dockershim using docker stats. This enables use
of the summary api to get container metrics on Windows where CRI stats
are enabled.