* {@code AsyncClosingFunction.apply}. Any heavyweight operations should occur in other threads
   * responsible for completing the returned {@code ClosingFuture}.)
   *
   * <p>After calling this method, you may not call {@link #finishToFuture()}, {@link
   * #finishToValueAndCloser(ValueAndCloserConsumer, Executor)}, or any other derivation method on
   * the original {@code ClosingFuture} instance.
   *