aggregator_feed_items_load() replaced with Aggregator storage controller methods

The aggregator_feed_items_load() function (also named aggregator_load_feed_items() for part of the 8.x development cycle) has been split up and moved to 3 separate methods in \Drupal\aggregator\ItemStorageControllerInterface.

In Drupal 7:

// Loads all feed items.
$items = aggregator_feed_items_load('sum');

// Loads feed items filtered by feed 'source'.  $feed object contains a fid property for the feed ID.
$items = aggregator_feed_items_load('source', $feed);

// Loads feed items filtered by feed category. $category array contains a cid key for the category ID.
$items = aggregator_feed_items_load('category', $category);

In Drupal 8

// Get the ItemStorageController from the dependency injection container.
$storage_controller = Drupal::entityManager()->getStorageController('aggregator_item');

// Loads all feed items.
$storage_controller->loadAll();

// Loads feed items by feed ID ($fid).
$storage_controller->loadByFeed($fid)

// Loads feed items by Category ID ($cid).
$storage_controller->loadByCategory($cid);