Change all references to the container ID in pkg/kubelet/... to the
strong type defined in pkg/kubelet/container: ContainerID
The motivation for this change is to make the format of the ID
unambiguous, specifically whether or not it includes the runtime
prefix (e.g. "docker://").
Add a new latency metric for the time from seeing the pod for the first time
to starting a pod worker for it.
Also, change PodStartLatency to include this initial processing latency.
