[refactor/grpc] Use of RouterServer interface instead of RouterClient in udp
[refactor/grpc] Use of RouterServer interface instead of RouterClient in udp

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