Support for saving and deleting revisions in the default storage controller and entity classes

The default Entity and DatabaseStorageController now support saving and deleting entity revisions.

To support this, a number of new methods have been added.

• EntityStorageControllerInterface::deleteRevision($revision_id)
  Deletes a specific revision. There is also a wrapper function called entity_revision_delete($entity_type, $revision_id)
• EntityInterface::setNewRevision($value = TRUE)
  Allows to set that an entity should be saved as a new revision. Replaces the node specific revision property.
• EntityInterface::isNewRevision()
  Corresponding function that returns TRUE if an entity should be saved as a revision. Usually only used by storage controllers and is reset after the revision was saved, just like EntityInterface::isNew(). To check if an entity was saved as a new revision in hook_entity_update(), compare the revision ids: $entity->getRevisionId() != $entity->original->getRevisionId() .
• The entity class can implement EntityInterface::preSaveRevision(EntityStorageControllerInterface $storage_controller, \stdClass $record) to change the revision record that is going to be saved.

Store a new as a new revision in 7.x

$node->revision = TRUE;
node_save($node);

8.x

$node->setNewRevision();
$node->save();

// Load a revision.
$node = \Drupal::entityTypeManager()->getStorage('node')->loadRevision(1);

// Delete a revision.
$node = \Drupal::entityTypeManager()->getStorage('node')->deleteRevision(1);