[ASDataController] Cancel if we lose our data source, fix bugs (#1987)

[ASRangeController] We're already on main thread, remove blocks

Make data source read-only, clarify what's asynchronous

[ASDataController] Clean up some interfaces

[ASDataController] A little more cleanup

[ASDataController] Cleanup

[ASDataController] Restore some changes, exit more often

[ASDataController] Use item counts that we already have rather than requerying them

[ASDataController] Revert weakifications

[ASDataController] Add a mechanism to measure how much work we avoided

AsyncDisplayKit/Details/ASCollectionDataController.h

@@ -40,6 +40,8 @@
 
 @interface ASCollectionDataController : ASChangeSetDataController
 
+- (instancetype)initWithDataSource:(id<ASCollectionDataControllerSource>)dataSource NS_DESIGNATED_INITIALIZER;
+
 - (ASCellNode *)supplementaryNodeOfKind:(NSString *)kind atIndexPath:(NSIndexPath *)indexPath;
 
 @end