[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/ASCollectionNode.mm

@@ -111,6 +111,7 @@
 {
   ASDN::RecursiveMutex _environmentStateLock;
   Class _collectionViewClass;
+  id<ASBatchFetchingDelegate> _batchFetchingDelegate;
 }
 @property (nonatomic) _ASCollectionPendingState *pendingState;
 @end
@@ -442,6 +443,16 @@
   return nil;
 }
 
+- (void)setBatchFetchingDelegate:(id<ASBatchFetchingDelegate>)batchFetchingDelegate
+{
+  _batchFetchingDelegate = batchFetchingDelegate;
+}
+
+- (id<ASBatchFetchingDelegate>)batchFetchingDelegate
+{
+  return _batchFetchingDelegate;
+}
+
 #pragma mark - Range Tuning
 
 - (ASRangeTuningParameters)tuningParametersForRangeType:(ASLayoutRangeType)rangeType