onHandshake(data) {
    this.emitReserved("handshake", data);
    this.id = data.sid;
    this.transport.query.sid = data.sid;
    this._pingInterval = data.pingInterval;
    this._pingTimeout = data.pingTimeout;
    this._maxPayload = data.maxPayload;
    this.onOpen();
    if ("closed" === this.readyState)
      return;
    this._resetPingTimeout();
  }
Called upon handshake completion.

@param {Object} data - handshake obj
@private