function send_next_request_batch(index) {
            var request_batch = request_batches[index],
                cb = function (errors, results) {
                    index += 1;
                    if (index === total_request_batches) {
                        callback && callback(errors, results);
                    } else {
                        send_next_request_batch(index);
                    }
                };

            async_all(request_batch, send_event_batch, cb);
        }
Asynchronously sends batches of requests
@param {number} index