send events in batches of 50 to reduce requests

[john-doherty]

Why -> #5 (comment)
How -> Mixpanel multi-event
Logic: something like this needs adding into the send() method before we turn them into requests

var temp = [];

/**
 * to reduce network requests, we batch up events in sequence into one request (50 per batch)
 * https://developer.mixpanel.com/reference/events#track-events-in-batch
 * event batching logic
 * --------------------
 * is this an event?
 *  - yes
 *      - was the previous request a batch with less than 50?
 *          - yes
 *              - append to previous request
 *          - no
 *              - is the next request an event?
 *                  - yes
 *                      - make this a batch request
 *                  - no
 *                      - process this event as normal
 *  - no
 *      - process this event as normal
 */
for (var i = 0, l = items.length; i < l; i++) {

    // is this an event?
    if (items[i].event) {

        // is the previous item an event batch with less than 50?
        if (temp[i - 1] && Array.isArray(temp[i - 1]) && temp[i - 1][0].event && temp[i - 1].length < 50) {

            // append to previous batch
            temp[i - 1].push(items[i]);
        }
        // is the next item an event?
        else if (temp[i + 1] && temp[i + 1].event) {

            // make this one a batch in prep for the next
            temp.push([items[i]]);
        }
        // otherwise, process as single event
        else {
            temp.push(items[i]);
        }
    }
    // otherwise, append as normal
    else {
        temp.push(items[i]);
    }
}

We also need to be mindful of how we clean up (remove) successful events posted to Mixpanel. At the moment, it's cleaning them up by going through the requests one at a time checking for __completed. That will no longer work if 50 events are sent at once