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.mm

@@ -131,7 +131,8 @@ static NSString *ASTextNodeTruncationTokenAttributeName = @"ASTextNodeTruncation
     _constrainedSize = CGSizeMake(-INFINITY, -INFINITY);
 
     // Placeholders
-    self.placeholderEnabled = YES;
+    // Disabled by default in ASDisplayNode, but add a few options for those who toggle
+    // on the special placeholder behavior of ASTextNode.
     _placeholderColor = ASDisplayNodeDefaultPlaceholderColor();
     _placeholderInsets = UIEdgeInsetsMake(1.0, 0.0, 1.0, 0.0);
   }
@@ -747,6 +748,8 @@ static NSString *ASTextNodeTruncationTokenAttributeName = @"ASTextNodeTruncation
 
 - (UIImage *)placeholderImage
 {
+  // FIXME: Replace this implementation with reusable CALayers that have .backgroundColor set.
+  // This would completely eliminate the memory and performance cost of the backing store.
   CGSize size = self.calculatedSize;
   UIGraphicsBeginImageContext(size);
   [self.placeholderColor setFill];