[Batch Fetching] Add ASBatchFetchingDelegate (#281)

* Add ASBatchFetchingDelegate
- In addition to checking remaining leading screens, ASBatchFetching now also calculates a remaining time and consults its delegate if needed.
- The delegate can override the decision of ASBatchFetching, for example based on remaining time and average time of past batch requests.

* Fix up tests

* Update CHANGELOG

Source/ASTableNode+Beta.h

@@ -0,0 +1,25 @@
+//
+//  ASTableNode+Beta.h
+//  Texture
+//
+//  Copyright (c) 2017-present, Pinterest, Inc.  All rights reserved.
+//  Licensed under the Apache License, Version 2.0 (the "License");
+//  you may not use this file except in compliance with the License.
+//  You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+
+#import <AsyncDisplayKit/ASTableNode.h>
+
+@protocol ASBatchFetchingDelegate;
+
+NS_ASSUME_NONNULL_BEGIN
+
+@interface ASTableNode (Beta)
+
+@property (nonatomic, weak) id<ASBatchFetchingDelegate> batchFetchingDelegate;
+
+@end
+
+NS_ASSUME_NONNULL_END