Watch m17 reflector activity and send transmittion notifications
to a MQTT broker. This version of the watcher monitors changes
in the mrefd.xml file for stations whose lastheard time is
newer than the last reported lastheard.
Copy env.sample to .env and edit that file to specify the
MQTT broker address and port.
Invoke as ./m17ref-watcher. While prototyping, this can be
done within a terminal multiplexer like tmux or screen.