[router] Write tests for local address keeper
[router] Write tests for local address keeper

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