[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/Details/ASBatchFetchingDelegate.h

@@ -0,0 +1,27 @@
+//
+//  ASBatchFetchingDelegate.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 <UIKit/UIKit.h>
+
+@protocol ASBatchFetchingDelegate <NSObject>
+
+/**
+ * @abstract Determine if batch fetching should begin based on the remaining time.
+ * If the delegate doesn't have enough information to confidently decide, it can take the given hint.
+ *
+ * @param remainingTime The amount of time left for user to reach the end of the scroll view's content.
+ *
+ * @param hint A hint for the delegate to fallback to.
+ */
+- (BOOL)shouldFetchBatchWithRemainingTime:(NSTimeInterval)remainingTime hint:(BOOL)hint;
+
+@end