code: function RequestManager (client, replyToTopic) {
    /**
     * The {@link Client} instance through which requests that this manager
     * tracks are made.
     * @type {Client}
     * @name RequestManager#_client
     * @private
     */
    this._client = client
    /**
     * The topic that the broker is expected to publish responses to for the
     * client.
     * @type {string}
     * @name RequestManager#_replyToTopic
     * @private
     */
    this.replyToTopic = replyToTopic
    /**
     * Object representing active requests. Each key contains the
     * {@link Message#messageId} of the request. Each corresponding value
     * contains the {@link Response} message received for the request, if one
     * has been received.
     * @type {Object}
     * @name RequestManager#_requests
     * @private
     */
    this._requests = {}
    client.addResponseCallback(replyToTopic, this.onResponse.bind(this), true)
  }
docs: @classdesc Manager that tracks outstanding requests and notifies the
  appropriate parties (invoking a response callback, notifying a waiting
  object, etc.) when a corresponding response is received.
  
  This purpose of this object is to collaborate with a {@link Client}
  instance.
  @private
  @param {Client} client - The {@link Client} instance through which requests
  that this manager tracks are made.
  @param {String} replyToTopic - The topic that the broker is expected to
  publish responses to for the client.
  @constructor