MQTT: WIP, re-organized disconnection handling, now keep track of connection status and reconnect accordingly.

This fixes bug with non-reconnecting to broker when having no published variables.

<?xml version='1.0' encoding='utf-8'?>
<ModbusServerNode xmlns:xsd="http://www.w3.org/2001/XMLSchema" Local_Port_Number="1502" Local_IP_Address="127.0.0.1"/>