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/RandomCoreGraphicsNode.m

@@ -0,0 +1,93 @@
+//
+//  RandomCoreGraphicsNode.m
+//  Sample
+//
+//  Created by Scott Goodson on 9/5/15.
+//  Copyright (c) 2015 Facebook. All rights reserved.
+//
+
+#import "RandomCoreGraphicsNode.h"
+#import <AsyncDisplayKit/ASDisplayNode+Subclasses.h>
+
+@implementation RandomCoreGraphicsNode
+
++ (UIColor *)randomColor
+{
+  CGFloat hue = ( arc4random() % 256 / 256.0 );  //  0.0 to 1.0
+  CGFloat saturation = ( arc4random() % 128 / 256.0 ) + 0.5;  //  0.5 to 1.0, away from white
+  CGFloat brightness = ( arc4random() % 128 / 256.0 ) + 0.5;  //  0.5 to 1.0, away from black
+  return [UIColor colorWithHue:hue saturation:saturation brightness:brightness alpha:1];
+}
+
++ (void)drawRect:(CGRect)bounds withParameters:(id<NSObject>)parameters isCancelled:(asdisplaynode_iscancelled_block_t)isCancelledBlock isRasterizing:(BOOL)isRasterizing
+{
+  CGFloat locations[3];
+  NSMutableArray *colors = [NSMutableArray arrayWithCapacity:3];
+  [colors addObject:(id)[[RandomCoreGraphicsNode randomColor] CGColor]];
+  locations[0] = 0.0;
+  [colors addObject:(id)[[RandomCoreGraphicsNode randomColor] CGColor]];
+  locations[1] = 1.0;
+  [colors addObject:(id)[[RandomCoreGraphicsNode randomColor] CGColor]];
+  locations[2] = ( arc4random() % 256 / 256.0 );
+
+  
+  CGContextRef ctx = UIGraphicsGetCurrentContext();
+  CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
+  CGGradientRef gradient = CGGradientCreateWithColors(colorSpace, (CFArrayRef)colors, locations);
+  
+  CGGradientDrawingOptions drawingOptions;
+  CGContextDrawLinearGradient(ctx, gradient, CGPointZero, CGPointMake(bounds.size.width, bounds.size.height), drawingOptions);
+  
+  CGColorSpaceRelease(colorSpace);
+}
+
+- (NSObject *)drawParametersForAsyncLayer:(_ASDisplayLayer *)layer
+{
+  return [self description];
+}
+
+- (NSDictionary *)textStyle
+{
+  UIFont *font = [UIFont fontWithName:@"HelveticaNeue" size:36.0f];
+  
+  NSMutableParagraphStyle *style = [[NSParagraphStyle defaultParagraphStyle] mutableCopy];
+  style.paragraphSpacing = 0.5 * font.lineHeight;
+  style.hyphenationFactor = 1.0;
+  
+  return @{ NSFontAttributeName: font,
+            NSParagraphStyleAttributeName: style };
+}
+
+- (instancetype)init
+{
+  if (!(self = [super init])) {
+    return nil;
+  }
+  
+  _textNode = [[ASTextNode alloc] init];
+  _textNode.placeholderEnabled = NO;
+  _textNode.attributedString = [[NSAttributedString alloc] initWithString:@"Hello, ASDK!"
+                                                               attributes:[self textStyle]];
+  [self addSubnode:_textNode];
+  
+  return self;
+}
+
+- (CGSize)calculateSizeThatFits:(CGSize)constrainedSize
+{
+  [_textNode measure:constrainedSize];
+  return CGSizeMake(constrainedSize.width, 100);
+}
+
+- (void)layout
+{
+  CGSize boundsSize = self.bounds.size;
+  CGSize textSize = _textNode.calculatedSize;
+  CGRect textRect = CGRectMake(roundf((boundsSize.width - textSize.width) / 2.0),
+                               roundf((boundsSize.height - textSize.height) / 2.0),
+                               textSize.width,
+                               textSize.height);
+  _textNode.frame = textRect;
+}
+
+@end