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.m

@@ -10,7 +10,13 @@
 #import "ASAssert.h"
 #import "ASDimension.h"
 
+// FIXME: Temporary nonsense import until method names are finalized and exposed
+#import "ASDisplayNode+Subclasses.h"
+
 @implementation ASViewController
+{
+  BOOL _ensureDisplayed;
+}
 
 - (instancetype)initWithNode:(ASDisplayNode *)node
 {
@@ -33,15 +39,25 @@
 
 - (void)viewWillLayoutSubviews
 {
+  [super viewWillLayoutSubviews];
   CGSize viewSize = self.view.bounds.size;
   ASSizeRange constrainedSize = ASSizeRangeMake(viewSize, viewSize);
   [_node measureWithSizeRange:constrainedSize];
-  [super viewWillLayoutSubviews];
+}
+
+- (void)viewDidLayoutSubviews
+{
+  if (_ensureDisplayed && self.neverShowPlaceholders) {
+    _ensureDisplayed = NO;
+    [self.node recursivelyEnsureDisplay];
+  }
+  [super viewDidLayoutSubviews];
 }
 
 - (void)viewWillAppear:(BOOL)animated
 {
   [super viewWillAppear:animated];
+  _ensureDisplayed = YES;
   [_node recursivelyFetchData];
 }