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/ASViewController.h

@@ -13,7 +13,11 @@
 
 @property (nonatomic, strong, readonly) ASDisplayNode *node;
 
-//TODO Use nonnull annotation late on. Travis doesn't recognize it (yet).
+// AsyncDisplayKit 2.0 BETA: This property is still being tested, but it allows
+// blocking as a view controller becomes visible to ensure no placeholders flash onscreen.
+// Refer to examples/SynchronousConcurrency, AsyncViewController.m
+@property (nonatomic, assign) BOOL neverShowPlaceholders;
+
 - (instancetype)initWithNode:(ASDisplayNode *)node;
 
 @end