* A special purpose queue/executor that dispatches listener events serially on a configured
   * executor. Each event can be added and dispatched as separate phases.
   *
   * <p>This class is very similar to {@link SequentialExecutor} with the exception that events can
   * be added without necessarily executing immediately.
   */
  private static final class PerListenerQueue<L> implements Runnable {
    final L listener;