Added EntityStorageControllerInterface::loadUnchanged() to load unchanged entities

The EntityStorageControllerInterface::loadUnchanged() has been added as the OO counterpart of entity_load_unchanged(), which now just forwards the call to the storage controller. As before the returned value is the entity as currently stored.

Before:

<?php
class ClassDealingWithEntities {

  public function doStuff(EntityInterface $entity) {
    // Bad, not properly unit-testable.
    $unchanged_entity = entity_load_unchanged($entity->id());
    // ...
  }

}
?>

After:

<?php
class ClassDealingWithEntities {

  protected $manager;

  public function __construct(EntityManagerInterface $manager) {
     $this->manager = $manager;
  }

  public function doStuff(EntityInterface $entity) {
    $unchanged_entity = $this->manager
      ->getStorageController($entity->entityType())
      ->loadUnchanged($entity->id());
    // ...
  }

}
?>