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.

examples/SynchronousConcurrency/Sample/AsyncViewController.m

@@ -0,0 +1,37 @@
+//
+//  AsyncViewController.m
+//  Sample
+//
+//  Created by Scott Goodson on 9/26/15.
+//  Copyright © 2015 Facebook. All rights reserved.
+//
+
+#import "AsyncViewController.h"
+#import "RandomCoreGraphicsNode.h"
+
+@implementation AsyncViewController
+
+- (instancetype)init
+{
+  if (!(self = [super initWithNode:[[RandomCoreGraphicsNode alloc] init]])) {
+    return nil;
+  }
+
+  self.neverShowPlaceholders = YES;
+  self.tabBarItem = [[UITabBarItem alloc] initWithTabBarSystemItem:UITabBarSystemItemFavorites tag:0];
+  return self;
+}
+
+- (void)viewWillAppear:(BOOL)animated
+{
+  // FIXME: This is only being called on the first time the UITabBarController shows us.
+  [super viewWillAppear:animated];
+}
+
+- (void)viewDidDisappear:(BOOL)animated
+{
+  [self.node recursivelyClearContents];
+  [super viewDidDisappear:animated];
+}
+
+@end