On iOS, don't refresh all cells when items are inserted/removed

[veyndan]

When any property of LazyList changes on iOS, we invoke collectionView.reloadData(). This is an expensive operation which shouldn't be necessary to call, as we are given atomic change updates within UICollectionViewListUpdateCallback.

Currently, I don't have a lot of confidence in UICollectionViewListUpdateCallback being given the correct atomic change updates. This is due to the lack of tests, and the fact that on Android we invoke notifyDatasetChanged (the equivalent to iOS' reloadData) on a specific code path, further reducing trust that these atomic change updates are correct.

Therefore, I would argue that this is blocked by:

• Test validity of atomic updates given by WindowedLazyList #1497
• Replace notifyDataSetChanged with atomic change events #1128

[swankjesse]

Implemented by #1538