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/Details/ASBasicImageDownloader.h

@@ -14,4 +14,6 @@
  */
 @interface ASBasicImageDownloader : NSObject <ASImageDownloaderProtocol>
 
++ (instancetype)sharedImageDownloader;
+
 @end