Make list pods constant time

This moves all status-computing logic to the pod cache, and also starts caching more things (like whether a node exists). Will make it easier in the future to move status computation into a separate process or all the way down to kubelet.

Fixes #2951

Fixes #2975

Improves #2922

I'm having troubles running e2e. Will figure that out tomorrow and also squash.
Please don't defer an anonymous function, I think it really hurts readability.

And in general this function is structured really weirdly.

How about extracting out a "constructNewStatus" function, and then only setting the status once at the end.