Track node lifecycle events

[drmorr0]

Right now, when sk-vnode comes up, the node is "instantly available" to accept pods (and, in addition, sk-vnode replica count changes almost instantly). In reality, cloud providers take some time to provision new nodes on scale-up, and nodes take some amount of time to bootstrap before they are ready to run pods. We should have some way of modeling or tracking this in our simulation to provide better results.

This is "planned work" but currently kept as a discussion since I don't know the best way to implement this yet.