* waiting for the running calls to complete.
   *
   * If more than [maxRequests] requests are in flight when this is invoked, those requests will
   * remain in flight.
   */
  var maxRequests = 64
    get() = this.withLock { field }
    set(maxRequests) {
      require(maxRequests >= 1) { "max < 1: $maxRequests" }
      this.withLock {
        field = maxRequests