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/AsyncTableViewController.m

@@ -0,0 +1,91 @@
+/* This file provided by Facebook is for non-commercial testing and evaluation
+ * purposes only.  Facebook reserves all rights not expressly granted.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+ * FACEBOOK BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
+ * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+#import <AsyncDisplayKit/AsyncDisplayKit.h>
+#import <AsyncDisplayKit/ASAssert.h>
+
+#import "AsyncTableViewController.h"
+#import "RandomCoreGraphicsNode.h"
+
+@interface AsyncTableViewController () <ASTableViewDataSource, ASTableViewDelegate>
+{
+  ASTableView *_tableView;
+}
+
+@end
+
+@implementation AsyncTableViewController
+
+#pragma mark -
+#pragma mark UIViewController.
+
+- (instancetype)init
+{
+  if (!(self = [super init]))
+    return nil;
+
+  _tableView = [[ASTableView alloc] initWithFrame:CGRectZero style:UITableViewStylePlain];
+  _tableView.separatorStyle = UITableViewCellSeparatorStyleNone;
+  _tableView.asyncDataSource = self;
+  _tableView.asyncDelegate = self;
+  
+  ASRangeTuningParameters tuningParameters;
+  tuningParameters.leadingBufferScreenfuls = 0.5;
+  tuningParameters.trailingBufferScreenfuls = 1.0;
+  [_tableView setTuningParameters:tuningParameters forRangeType:ASLayoutRangeTypePreload];
+  [_tableView setTuningParameters:tuningParameters forRangeType:ASLayoutRangeTypeRender];
+  
+  self.tabBarItem = [[UITabBarItem alloc] initWithTabBarSystemItem:UITabBarSystemItemFeatured tag:0];
+  self.navigationItem.rightBarButtonItem = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemRedo
+                                                                                         target:self
+                                                                                         action:@selector(reloadEverything)];
+
+  return self;
+}
+
+- (void)reloadEverything
+{
+  [_tableView reloadData];
+}
+
+- (void)viewDidLoad
+{
+  [super viewDidLoad];
+
+  [self.view addSubview:_tableView];
+}
+
+- (void)viewWillLayoutSubviews
+{
+  _tableView.frame = self.view.bounds;
+}
+
+- (BOOL)prefersStatusBarHidden
+{
+  return YES;
+}
+
+#pragma mark -
+#pragma mark ASTableView.
+
+- (ASCellNode *)tableView:(ASTableView *)tableView nodeForRowAtIndexPath:(NSIndexPath *)indexPath
+{
+  RandomCoreGraphicsNode *elementNode = [[RandomCoreGraphicsNode alloc] init];
+  elementNode.preferredFrameSize = CGSizeMake(320, 100);
+  return elementNode;
+}
+
+- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
+{
+  return 100;
+}
+
+@end