expose relayoutAllNodes at ASCollectionView level (#1683)

* expose relayoutAllNodes at ASCollectionView level

* rename relayoutAllNodes to relayoutItems to be more UIKit centric

AsyncDisplayKit/ASTableView.h

@@ -134,6 +134,12 @@ NS_ASSUME_NONNULL_BEGIN
  */
 - (void)reloadDataImmediately;
 
+/**
+ * Triggers a relayout of all nodes.
+ *
+ */
+- (void)relayoutItems;
+
 /**
  *  begins a batch of insert, delete reload and move operations. This method must be called from the main thread.
  */