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 think this code proves my point about the pattern being borked.  Don't you call ```p.lock.Lock()``` twice here without an Unlock in between?  (L85 and L94)

Am I missing something?