/** Sends any pending long-polling responses after a state change. */
function sendLongPollResponses(game) {
    let polls = longpolls[game.id]
    delete longpolls[game.id]  // remove
    if (polls) {
        for (let response of polls) {
            // a response object is a list with the actual HTTP response and the userid.
            let res = response[0]
            let userid = response[1]
            console.log(`End long-poll request for ${game.id} by ${userid}`)
            res.json(toJson(game, userid))
            res.end()
        }
    }
}