Add -waitUntilAllUpdatesAreCommitted to ASTableView and ASCollectionView

The API allows consumer of ASTableView or ASCollectionCiew to block execution of the main thread until all section and row updates are committed.

AsyncDisplayKit/ASCollectionView.mm

@@ -268,6 +268,12 @@ static NSString * const kCellReuseIdentifier = @"_ASCollectionViewCell";
   [super reloadData];
 }
 
+- (void)waitUntilAllUpdatesAreCommitted
+{
+  ASDisplayNodeAssertMainThread();
+  [_dataController waitUntilAllUpdatesAreCommitted];
+}
+
 - (void)setDataSource:(id<UICollectionViewDataSource>)dataSource
 {
   // UIKit can internally generate a call to this method upon changing the asyncDataSource; only assert for non-nil.