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.mm

@@ -17,6 +17,9 @@
 #import "UICollectionViewLayout+ASConvenience.h"
 #import "ASInternalHelpers.h"
 
+// FIXME: Temporary nonsense import until method names are finalized and exposed
+#import "ASDisplayNode+Subclasses.h"
+
 const static NSUInteger kASCollectionViewAnimationNone = UITableViewRowAnimationNone;
 
 
@@ -315,6 +318,16 @@ static BOOL _isInterceptedSelector(SEL sel)
   return [[_dataController nodeAtIndexPath:indexPath] calculatedSize];
 }
 
+- (ASCellNode *)nodeForItemAtIndexPath:(NSIndexPath *)indexPath
+{
+  return [_dataController nodeAtIndexPath:indexPath];
+}
+
+- (NSIndexPath *)indexPathForNode:(ASCellNode *)cellNode
+{
+  return [_dataController indexPathForNode:cellNode];
+}
+
 - (NSArray *)visibleNodes
 {
   NSArray *indexPaths = [self indexPathsForVisibleItems];
@@ -392,11 +405,6 @@ static BOOL _isInterceptedSelector(SEL sel)
   [_dataController moveRowAtIndexPath:indexPath toIndexPath:newIndexPath withAnimationOptions:kASCollectionViewAnimationNone];
 }
 
-- (ASCellNode *)nodeForItemAtIndexPath:(NSIndexPath *)indexPath
-{
-  return [_dataController nodeAtIndexPath:indexPath];
-}
-
 #pragma mark -
 #pragma mark Intercepted selectors.
 
@@ -490,6 +498,11 @@ static BOOL _isInterceptedSelector(SEL sel)
   if ([_asyncDelegate respondsToSelector:@selector(collectionView:willDisplayNodeForItemAtIndexPath:)]) {
     [_asyncDelegate collectionView:self willDisplayNodeForItemAtIndexPath:indexPath];
   }
+  
+  ASCellNode *cellNode = [self nodeForItemAtIndexPath:indexPath];
+  if (cellNode.neverShowPlaceholders) {
+    [cellNode recursivelyEnsureDisplay];
+  }
 }
 
 - (void)collectionView:(UICollectionView *)collectionView didEndDisplayingCell:(UICollectionViewCell *)cell forItemAtIndexPath:(NSIndexPath *)indexPath