Refactor Kubelet's server functionality into a server package. Most
notably, move pkg/kubelet/server.go into
pkg/kubelet/server/server.go. This will lead to better separation of
concerns and a more readable code hierarchy.
This leaves `pkg/kubelet/server/server.go` looking a little ugly as there is an extra layer of "config" structs that isn't needed. This is left as a TODO for now.