_setupConnectionTimeout () {
    const timeout = this._config.connectionTimeout
    if (timeout) {
      const webSocket = this._ws

      return setTimeout(() => {
        if (webSocket.readyState !== WS_OPEN) {
          this._connectionTimeoutFired = true
          webSocket.close()
        }
      }, timeout)
    }
    return null
  }
Set connection timeout on the given WebSocket, if configured.
@return {number} the timeout id or null.
@private