Separate TTL for newly created stacks

[aermakov-zalando]

#71 implemented a hacky solution to a problem where the controller scaled down stacks before the deployment even had a chance to switch traffic to those. However, this behaviour is not exactly intuitive, and could lead to deployments being kept alive for longer than necessary if the user quickly switches traffic back and forth. A cleaner approach would be to mark newly created stacks with an annotation that's erased when they get traffic and introducing a separate TTL for stacks in this state.