in mqtt-service.h:
-> add a number max of topic to subscribe

in mqtt-service.c:
-> add the management of multiple topics to subscribe
-> add client_id in the topic name when registering
	ex: 00124B0002CB885C/led
-> handle when we receive led topic
TODO: match with topic id

in example.c:
-> start process neighbor for waiting when a neighbor is discovered.
In this case we can stop neighbor process and start the app
-> add mac address for client id