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.
This is just moving preexisting behavior. I'd prefer to change ip caching semantics in another PR, since that's not the focus of this PR.