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/ASTableView.h

@@ -27,8 +27,28 @@
  */
 @interface ASTableView : UITableView
 
-@property (nonatomic, weak) id<ASTableViewDataSource> asyncDataSource;
 @property (nonatomic, weak) id<ASTableViewDelegate> asyncDelegate;      // must not be nil
+@property (nonatomic, weak) id<ASTableViewDataSource> asyncDataSource;
+
+/**
+ * Initializer.
+ *
+ * @param frame A rectangle specifying the initial location and size of the table view in its superview’€™s coordinates.
+ * The frame of the table view changes as table cells are added and deleted.
+ *
+ * @param style A constant that specifies the style of the table view. See UITableViewStyle for descriptions of valid constants.
+ *
+ * @param asyncDataFetchingEnabled This option is reserved for future use, and currently a no-op.
+ *
+ * @discussion If asyncDataFetching is enabled, the `ASTableView` will fetch data through `tableView:numberOfRowsInSection:` and
+ * `tableView:nodeForRowAtIndexPath:` in async mode from background thread. Otherwise, the methods will be invoked synchronically
+ * from calling thread.
+ * Enabling asyncDataFetching could avoid blocking main thread for `ASCellNode` allocation, which is frequently reported issue for
+ * large scale data. On another hand, the application code need take the responsibility to avoid data inconsistence. Specifically,
+ * we will lock the data source through `tableViewLockDataSource`, and unlock it by `tableViewUnlockDataSource` after the data fetching.
+ * The application should not update the data source while the data source is locked, to keep data consistence.
+ */
+- (instancetype)initWithFrame:(CGRect)frame style:(UITableViewStyle)style asyncDataFetching:(BOOL)asyncDataFetchingEnabled;
 
 /**
  * Tuning parameters for a range.
@@ -50,26 +70,6 @@
  */
 - (void)setTuningParameters:(ASRangeTuningParameters)tuningParameters forRangeType:(ASLayoutRangeType)rangeType;
 
-/**
- * Initializer.
- *
- * @param frame A rectangle specifying the initial location and size of the table view in its superview’€™s coordinates. 
- * The frame of the table view changes as table cells are added and deleted.
- *
- * @param style A constant that specifies the style of the table view. See UITableViewStyle for descriptions of valid constants.
- *
- * @param asyncDataFetchingEnabled Enable the data fetching in async mode.
- * 
- * @discussion If asyncDataFetching is enabled, the `ASTableView` will fetch data through `tableView:numberOfRowsInSection:` and
- * `tableView:nodeForRowAtIndexPath:` in async mode from background thread. Otherwise, the methods will be invoked synchronically 
- * from calling thread.
- * Enabling asyncDataFetching could avoid blocking main thread for `ASCellNode` allocation, which is frequently reported issue for 
- * large scale data. On another hand, the application code need take the responsibility to avoid data inconsistence. Specifically, 
- * we will lock the data source through `tableViewLockDataSource`, and unlock it by `tableViewUnlockDataSource` after the data fetching. 
- * The application should not update the data source while the data source is locked, to keep data consistence.
- */
-- (instancetype)initWithFrame:(CGRect)frame style:(UITableViewStyle)style asyncDataFetching:(BOOL)asyncDataFetchingEnabled;
-
 /**
  * The number of screens left to scroll before the delegate -tableView:beginBatchFetchingWithContext: is called.
  *