[broker] Add dump-implementation of Router.HandleDown()
[broker] Add dump-implementation of Router.HandleDown()

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