MQTT: WIP, PLC starts even if MQTT client can't connect broker, and publish+subscribe again when reconnecting.

MQTT: WIP, destroys paho client during failure handling at init and at cleanup. Use mutex to protect retrieve.

MQTT: Implements reconnecting in publish thread in case thread is waken-up but client is disconnected.

Note: paho's lostConnection callback got already disabled in previous commit.

MQTT: take C part away from python code for readability.

C lines change a bit because of unnecessary escaping for '\'.