The Supervisor generates this event every time it receives a status update from
the stem, meaning a node got created or re-created. A corresponding
SupervisorControl::node_status event relays the same information for users
interacting with the Supervisor over Broker.
The supervisor now starts listening on the configured Broker default address and
port when the new boolean SupervisorControl::enable_listen is T. Listening
remains disabled by default. Listening allows nodes to communicate with the
supervisor via the events laid out in control.zeek, to conduct further node
management.
