Sync pods asynchronously in the Kubelet.

This makes two main changes:
- Runs syncPod in a separate Go routine (and enforces only one of those
  runs at a time).
- Uses the pod list to determine if a container should be running or
  should be killed (used to use the output of syncPod).

Since Docker pulls are synchronized by the Docker daemon we still block
on that, but pods can now be removed and prepared for starting without
blocking on long pulls.
This won't kill duplicate containers started at different times.  In the current code, only containers that correspond to the most recent container id of a running pod are used.  I'm not sure how you could get into this state, but in this new logic if you have:

    pod-A-container-A-execution-1
    pod-A-container-A-execution-2

running at the same time, the kill loop will not remove either 1 or 2.  The algorithm should guarantee that only the later execution of 2 should be running at any one time.