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/ASTableView.h

@@ -163,6 +163,11 @@ NS_ASSUME_NONNULL_BEGIN
  */
 - (void)endUpdatesAnimated:(BOOL)animated completion:(void (^ _Nullable)(BOOL completed))completion;
 
+/**
+ *  Blocks execution of the main thread until all section and row updates are committed. This method must be called from the main thread.
+ */
+- (void)waitUntilAllUpdatesAreCommitted;
+
 /**
  * Inserts one or more sections, with an option to animate the insertion.
  *