[simulators.gateway] Simplify and fix communication system between goroutines
[simulators.gateway] Simplify and fix communication system between goroutines

mgranberry/ttn,jvanmalder/ttn,mgranberry/ttn,mgranberry/ttn,LoRaWanSoFa/ttn,TheThingsNetwork/ttn,jvanmalder/ttn,jvanmalder/ttn,TheThingsNetwork/ttn,LoRaWanSoFa/ttn,TheThingsNetwork/ttn