Add wait method to ASCollectionNode (#2416)

AsyncDisplayKit/ASCollectionNode.h

@@ -159,6 +159,11 @@ NS_ASSUME_NONNULL_BEGIN
  */
 - (void)performBatchUpdates:(nullable __attribute((noescape)) void (^)())updates completion:(nullable void (^)(BOOL finished))completion;
 
+/**
+ *  Blocks execution of the main thread until all section and item updates are committed to the view. This method must be called from the main thread.
+ */
+- (void)waitUntilAllUpdatesAreCommitted;
+
 /**
  * Inserts one or more sections.
  *