cp: Initial control plane implementation
Initial implementation lacks the following features:
- depool_threshold
- Prometheus metrics
- Ability to react to etcd events
- Ability to react to healthcheck results
- Retries on grpc call errors to other services
control plane initialization stage is implemented and performs the following actions:
- fetch the current list of realservers from etcd
- configure hcforwarder
- configure healthchecks
- configure forwarding plane
This MR ships the commits from !41 (merged), !42 (merged) and !43 (merged)