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.
I don't like this pattern.  Please use explicit locking.