Fix retain cycle in basic, cache-less ASNetworkImageNodes

This should not directly affect more complex apps that don't use the ASBasicImageDownloader.

Also disables the default-on text placeholders, as they churn memory during the measurement pass.
These were intended to be written with pure layers (without using backing stores), so I don't
think it is a reasonable default-on behavior until that is fixed.

AsyncDisplayKit/ASTextNode.h

@@ -79,6 +79,15 @@ typedef NS_ENUM(NSUInteger, ASTextNodeHighlightStyle) {
 
 #pragma mark - Placeholders
 
+/**
+ * @abstract ASTextNode has a special placeholder behavior when placeholderEnabled is YES.
+ *
+ * @discussion Defaults to NO.  When YES, it draws rectangles for each line of text,
+ * following the true shape of the text's wrapping.  This visually mirrors the overall
+ * shape and weight of paragraphs, making the appearance of the finished text less jarring.
+ */
+@property (nonatomic, assign) BOOL placeholderEnabled;
+
 /**
  @abstract The placeholder color.
  */