Implementation of Synchronous Concurrency features for AsyncDisplayKit 2.0

This provides internal features on _ASAsyncTransaction and ASDisplayNode to facilitate
implementing public API that allows clients to choose if they would prefer to block
on the completion of unfinished rendering, rather than allow a placeholder state to
become visible.

The internal features are:
-[_ASAsyncTransaction waitUntilComplete]
-[ASDisplayNode recursivelyEnsureDisplay]

Also provided are two such implementations:
-[ASCellNode setNeverShowPlaceholders:], which integrates with both Tables and Collections
-[ASViewController setNeverShowPlaceholders:], which should work with Nav and Tab controllers.

Lastly, on ASDisplayNode, a new property .shouldBypassEnsureDisplay allows individual node types
to exempt themselves from blocking the main thread on their display.

By implementing the feature at the ASCellNode level rather than ASTableView & ASCollectionView,
developers can retain fine-grained control on display characteristics.  For example, certain
cell types may be appropriate to display to the user with placeholders, whereas others may not.

Follow-up work will include unit tests, revisiting names, and the header locations of definitions.

AsyncDisplayKit/ASCollectionView.h

@@ -212,6 +212,15 @@
  */
 - (ASCellNode *)nodeForItemAtIndexPath:(NSIndexPath *)indexPath;
 
+/**
+ * Similar to -indexPathForCell:.
+ *
+ * @param cellNode a cellNode part of the table view
+ *
+ * @returns an indexPath for this cellNode
+ */
+- (NSIndexPath *)indexPathForNode:(ASCellNode *)cellNode;
+
 /**
  * Similar to -visibleCells.
  *