KAFKA-1690, we are adding the SSL support at Selector. However, there are still a few places where we use BlockingChannel for inter-broker communication. We need to replace those usage with Selector/NetworkClient to enable inter-broker communication over SSL. Specially, BlockingChannel is currently used in the following places.
1. ControllerChannelManager: for the controller to propagate metadata to the brokers.
2. KafkaServer: for the broker to send controlled shutdown request to the controller.
AbstractFetcherThread: for the follower to fetch data from the leader (through SimpleConsumer) moved to KAFKA-2440