Add functionality to safely close broker

- Force killing a broker causes Zookeeper to throw an error later because the ephemeral node connection was not properly shut down, as described [here](https://github.com/wurstmeister/kafka-docker/issues/389) and [here](https://stackoverflow.com/questions/39759071/error-while-starting-kafka-broker/44426034).

- Find the programmatic equivalent of pressing Ctrl + C in a terminal window