This is the keystone commit of turning MediaTileView asynchronous.
Because updates to the collection view's datasource now happen asynchronously and out-of-order, the batch update must be computed on the main queue based on what has changed.
MediaGalleryCollectionViewUpdater combines a log of operations from JournalingOrderedDictionary with the previous state of the collection view to compute insertions/deletions/reloads of items and sections.
