[ASTableView] Add constrainedSizeForRowAtIndexPath: to control row heights from delegate (#1769)

* [ASTableView] constrainedSizeForRowAtIndexPath

* Quick fix to header file

* Switch to Delegate from DataSource.

* Update testing variables to reflect switch to delegate

AsyncDisplayKit/ASTableView.h

@@ -433,6 +433,18 @@ NS_ASSUME_NONNULL_BEGIN
  */
 - (BOOL)shouldBatchFetchForTableView:(ASTableView *)tableView;
 
+/**
+ * Provides the constrained size range for measuring the row at the index path.
+ * Note: the widths in the returned size range are ignored!
+ * 
+ * @param tableView The sender.
+ *
+ * @param indexPath The index path of the node.
+ *
+ * @returns A constrained size range for layout the node at this index path.
+ */
+- (ASSizeRange)tableView:(ASTableView *)tableView constrainedSizeForRowAtIndexPath:(NSIndexPath *)indexPath;
+
 /**
  * Informs the delegate that the table view did remove the node which was previously
  * at the given index path from the view hierarchy.