support scale down

[jstrachan]

if we scale down the brokers, we should be able to use kubernetes lifecycle hooks - the preStop hook to ensure the pod that is being scaled down has all its destinations moved to another broker and has all its destinations drained before it lets kubernetes kill the pod.

So mostly the preStop hook needs to:

• mark the pod as stopping, tell the message-gateway to move the pods destinations to other brokers
• when that is complete, wait for the destinations to be drained in the pod