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.
Wow, go supports arbitrary types as keys? when did that happen?

...but maybe this is better as a map of container name to podFullName?