If a static pod changes, delete the corresponding mirror pod. When kubelet
could not see mirror pod from the API server update, it'd attemp to create a
new mirror pod with up-to-date specs.
This change cleans up the pod manager extensively so that
* Mirror pods are actually stored in the pod manager.
* Both (non-mirror) pods and mirror pods are indexed by UID and full name for
easy lookup and mapping. This is required for the next change to send
full pod along with the pod status update.
This change also renames mirrorManager as mirrorClient since it is merely a
client to contact the API server and create/delete mirror pods.
