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.
IMO, not calling Unlock via defer is a deadlock disaster waiting to happen, so you're not going to see any raw Unlock calls from me. I can move to a function, though.