When sending out an pod status update, kubelet

GETs the pod from the apiserver
Terminates if the apiserver returns an not found error; otherwise, proceed to
to update.
Even after a pod has been deleted, there might still be queued up updates for
the pod. This leads to expensive, unncessary GET operations. The situation is
worse when there are batch creation/deletion of a significant number of pods
(e.g., E2E tests), leaving many updates in the queue.

This change checks whether a pod exists before GET the pod from the apiserver
to avoid redundant GETs.