feature : One MQTT connection per gateway

[aaalloc]

During a simulation, one MQTT connection is created for all devices / gateways, they maybe are reason to this but it would be helpful to have the possibility to create one MQTT connection per gateways.

This way, a more realistic simulation is happening and other cases can be tested, for example if current chirpstack service (HA or mono structure) handle correctly deduplication from multiple gateways.