channel-js: ensure lastEventId is an integer, and correctly set lastAckedEvent
channel-js: ensure lastEventId is an integer, and correctly set lastAckedEvent

urbit/urbit,urbit/urbit,ngzax/urbit,urbit/urbit,ngzax/urbit,ngzax/urbit,urbit/urbit,urbit/urbit,ngzax/urbit,ngzax/urbit,ngzax/urbit,ngzax/urbit,urbit/urbit,urbit/urbit