structure.txt

user_init()  - entry point and setup
    initSysCfg() - setup configuration

    -- Init devices --
    *_init()

sys_init_complete() - Called when system_init complete
    - sets up callbacks for MQTT
    - sets up MQTT client
    
    - reports voltage
    
    - Starts WiFi connecton

wifiConnectCb()
    Connects to MQTT broker
    Starts NTP, starts timer to call getCurrentTime()
    or
    Disconnects from MQTT for status != STATION_GOT_IP
    

mqttConnectedCb()
    Starts device readings
    *_start()

mqttPublishedCb()
    *_is_complete() - to tell when devices have sent measurements

    State machine waits for MQTT messages to complete and then
    publishes a timestamp. When that drains it shuts down the system.

mqttDisconnectedCb()
    Simply reports

mqttDataCb()
    Called if MQTT message receive, but no subscriptions so not actually
    used.
    
user_deep_sleep()
    *_shutdown()
    Goto to lowest power mode and sleep