Introduce placeholders (#lazy_builder) to replace #post_render_cache

1. Added #lazy_builder callbacks, which are given only arguments that may contain only primitive types (string, int, bool, float, NULL), not objects.
2. Added #create_placeholder, which allows a render array to be forced to be placeholdered.
   In a follow-up issue, automatic placeholdering will be introduced. (Basically: if max-age = 0 or high-cardinality cache contexts are present for the subtree, set #create_placeholder = TRUE.)
3. Removed #post_render_cache (the above is its successor).
4. Removed RendererInterface::generateCachePlaceholder() and drupal_render_cache_generate_placeholder() because they are now obsolete.

A new general recommendation

• A #lazy_builder can build a render array from scratch. It is given only the arguments that you provide it, and that context may contain only primitive types (string, bool, int, float, NULL).
• When specifying #cache, provide a #lazy_builder to build out the entire render array in case of a cache miss.
• Together, the two above points allow Drupal to optimize: it is able to determine based on how dynamic the contents of the #builder callback are, whether the #lazy_builder (and thus the associated render array) should be rendered during the initial page load, or via BigPipe or ESI.
• Arguments should only be passed from an object that is already provided to a parent element. Otherwise, if the render array is standalone (not a child of any parent element) then the arguments should be retrieved inside the callback itself.

Concrete changes

Before

            $callback = 'comment.post_render_cache:renderForm';
            $context = array(
              'entity_type' => $entity->getEntityTypeId(),
              'entity_id' => $entity->id(),
              'field_name' => $field_name,
              'comment_type' => $this->getFieldSetting('comment_type'),
            );
            $placeholder = drupal_render_cache_generate_placeholder($callback, $context);
            $output['comment_form'] = array(
              '#post_render_cache' => array(
                $callback => array(
                  $context,
                ),
              ),
              '#markup' => $placeholder,
            );

  public function renderForm(array $element, array $context) {
    $values = array(
      'entity_type' => $context['entity_type'],
      'entity_id' => $context['entity_id'],
      'field_name' => $context['field_name'],
      'comment_type' => $context['comment_type'],
      'pid' => NULL,
    );
    $comment = $this->entityManager->getStorage('comment')->create($values);
    $form = $this->entityFormBuilder->getForm($comment);
    $markup = $this->renderer->render($form);

    $callback = 'comment.post_render_cache:renderForm';
    $placeholder = $this->generatePlaceholder($callback, $context);
    $element['#markup'] = str_replace($placeholder, $markup, $element['#markup']);
    $element = $this->renderer->mergeBubbleableMetadata($element, $form);

    return $element;
  }

After

            $output['comment_form'] = [
              '#lazy_builder' => ['comment.lazy_builders:renderForm', [
                $entity->getEntityTypeId(),
                $entity->id(),
                $field_name,
                $this->getFieldSetting('comment_type'),
              ]],
              '#create_placeholder' => TRUE,
            ];

  public function renderForm($commented_entity_type_id, $commented_entity_id, $field_name, $comment_type_id) {
    $values = array(
      'entity_type' => $commented_entity_type_id,
      'entity_id' => $commented_entity_id,
      'field_name' => $field_name,
      'comment_type' => $comment_type_id,
      'pid' => NULL,
    );
    $comment = $this->entityManager->getStorage('comment')->create($values);
    return $this->entityFormBuilder->getForm($comment);
  }