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.h

@@ -165,6 +165,11 @@ NS_ASSUME_NONNULL_BEGIN
  */
 - (void)reloadDataImmediately;
 
+/**
+ *  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;
+
 /**
  * Registers the given kind of supplementary node for use in creating node-backed supplementary views.
  *