Introduce ASIndexedNodeContext

- It is a container object that holds enough information to construct and measure a cell node
- All information is gathered on main thread. This allows ASDataController to capture the correct state of its data source before going to background.

AsyncDisplayKit/Details/ASIndexedNodeContext.h

@@ -0,0 +1,22 @@
+//
+//  ASIndexedNodeContext.h
+//  AsyncDisplayKit
+//
+//  Created by Huy Nguyen on 2/28/16.
+//  Copyright © 2016 Facebook. All rights reserved.
+//
+
+#import <AsyncDisplayKit/ASDataController.h>
+#import <AsyncDisplayKit/ASDimension.h>
+
+@interface ASIndexedNodeContext : NSObject
+
+@property (nonatomic, readonly, strong) ASCellNodeBlock nodeBlock;
+@property (nonatomic, readonly, assign) NSIndexPath *indexPath;
+@property (nonatomic, readonly, assign) ASSizeRange constrainedSize;
+
+- (instancetype)initWithNodeBlock:(ASCellNodeBlock)nodeBlock
+                        indexPath:(NSIndexPath *)indexPath
+                  constrainedSize:(ASSizeRange)constrainedSize;
+
+@end