Poison model healthcheck on shutdown
We have a problem in production where a broken model is not correctly
shutting down when requested, which means that director comes back up,
sees a healthy model (status READY/BUSY) and starts sending it new
predictions, even though it's supposed to be shutting down.
For now, try and improve the situation by poisoning the model
healthcheck on shutdown. This doesn't solve the underlying problem but
it should stop us losing more predictions to a known-broken pod.
