Pods can now be cordoned before a deploy

You can now cordon a pod so the proxy stops sending it new connections while existing ones drain. Rolling and blue-green deploys use this automatically; you can also do it by hand:

lark pod cordon 3d8e1f2a9c4b70
lark pod uncordon 3d8e1f2a9c4b70

Cordoned pods show up with a cordoned state in lark status.