code: function MongodbQueue(name, metrics, lock, opts) {
    if (!name) {
      throw new Error('name is required to create a mongodb queue');
    }
    if (!opts || !opts.mongodb) {
      throw new Error('mongodb is not specified to create mongodb queue');
    }
    this.queueName = name;
    this.metrics = metrics;
    this.lock = lock;
    this.lockName = opts.lockName || ('lock:sync:' + this.queueName);
    this.lockTimeout = opts.lockTimeout || 10000;
    this.mongodb = opts.mongodb;
    this.queueOptions = {
      visibility: opts.visibility || 30,
      ttl: opts.queueMessagesTTL || 24*60*60
    };
    this.queue;
  }
docs: construct a new queue that is backed by mongodb.
  @param {String} name the name of the queue.
  @param {Object} metrics an instance of fh-component-metrics client
  @param {Object} lock an instance of the lock service
  @param {Object} opts configuration options for the queue
  @param {Object} opts.mongodb an instance of the mongodb connection. This is required.
  @param {Number} opts.visibility see https://github.com/chilts/mongodb-queue#visibility---message-visibility-window
  @param {Object} opts.queueMessagesTTL The TTL (time to live) value for the messages on the queue. Default to 24 hours.