constructor() {
    this.lastHandleId_ = 0;
    this.lastWorldId_ = 0;
    this.worlds_ = new Map();
    this.idToHandle_ = new Map();
    this.lastMessageId_ = 0;
    this.callbacks_ = new Map();

    this.worldId_ = '.';
    this.sendToParent_ = null;
    this.cookieResponseCallbacks_ = new Map();
    this.debug_ = false;
  }
Main Rpc object. Keeps all the book keeping and performs message routing
between handles beloning to different worlds. Each 'world' has a singleton
'rpc' instance.