Extend Healthcheck with mqtt connection

[rtrive]

Summary

Add check to see if MQTT is available and it works properly. It will be nice to have on both application server and network server.

What is the use-case?

If for some reasons MQTT connection is not available, the system is not able to communicate with devices and this make LoRa network down

Implementation description

Suggestion: on healthcheck send a message to MQTT on a special topic and read this message. If, after, 5 seconds this message is not arrivate made application and network server unhealthy