send player counts, local and expected. Based on these the relay
accepts connections, declares the game full and ready for message
forwarding, and decides whether to accept a reconnect.
scheme where cookie is used only to connect, and is replaced for
reconnects by a relay-generated name that's supposed to be unique
across all games on all relays; let relay assign non-servers' hostIDs
rather than doing 'em randomly; use hostIDs for comms-level protocol's
channelNo where possible to avoid tripping over duplicate messages