Rollout Strategy: Blue/Green

[jlegrone]

Implement a rollout strategy where the controller waits for the latest version of the worker to be available (has minimum running replicas) and then updates build ID assignment rules to make the new version the default.

This is a strict superset of the functionality described in #2; the controller is also responsible for scaling deployments for reachable build IDs, and deleting deployments for unreachable build IDs.