* Release any resources held by this dispatcher. Any requests that are currently being dispatched
   * should return immediately. Responses returned after shutdown will not be transmitted: their
   * socket connections have already been closed.
   */
  open fun shutdown() {}

   * callback has returned. Synchronous calls become idle once [execute()][Call.execute] returns.
   * This means that if you are doing synchronous calls the network layer will not truly be idle
   * until every returned [Response] has been closed.
   */
  var idleCallback: Runnable? = null
    get() = this.withLock { field }
    set(value) {
      this.withLock { field = value }
    }

  private var executorServiceOrNull: ExecutorService? = null