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.
I've come around to the perspective that structuring around defer makes things easier to reason about, and only critical performance sensitive loops need to avoid the defer allocation cost.  I'd characterize performance sensitive loops as 1000/second, which I don't think sync will ever hit.