Creates a broker with no traffic.
Whether the broker is in reach. Set it to false to put every link on it
out of range at once, links a ladder owns included: they fail to connect,
send, or subscribe until it is true again, keeping their connections and
filters through the outage.
Takes the broker out of reach, or brings it back.
Creates a link to this broker, for driving directly.
An in-process broker.
Every transport built from one broker shares its traffic, so a message one publishes reaches the others that subscribed to the topic.