[router] Fix unfortunate race-condition with broker registration
[router] Fix unfortunate race-condition with broker registration

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