enqueue = (queue, item) => {
        // drop msgs from front of queue if size is going to be exceeded
        if (queue.length === msgQueueSize) { queue.shift(); }
        queue.push(item);
        return queue;
    }
Enqueue `item` in `queue`
@param {Denque} queue - Queue
@param {*} item - Item to enqueue
@private
@returns {Denque} `queue`