chat: cap maximum backlog size at 1000
chat: cap maximum backlog size at 1000

Caps maximum unread backlog that chat will fetch at 1000 messages.

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