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/Details/ASDataController.h

@@ -181,6 +181,8 @@ FOUNDATION_EXPORT NSString * const ASDataControllerRowNodeKind;
 
 - (void)reloadDataImmediatelyWithAnimationOptions:(ASDataControllerAnimationOptions)animationOptions;
 
+- (void)waitUntilAllUpdatesAreCommitted;
+
 /** @name Data Querying */
 
 - (NSUInteger)numberOfSections;