[ASDisplayNode] Always return the thread-safe cornerRadius property, even in slow CALayer rounding mode (#749)

- Failing to do so will introduce race conditions in which the property was updated on a background thread but main thread has not executed the block that updates the property of the node's layer. During that window, the layer's property is out-of-date and can't be used.
- After this change, ASDisplayNode's cornerRadius is the only source of truth and users must always use it instead of CALayer's.

Source/ASDisplayNode.h

@@ -659,6 +659,12 @@ extern NSInteger const ASDefaultDrawingPriority;
  * @default ASCornerRoundingTypeDefaultSlowCALayer
  */
 @property (nonatomic, assign)           ASCornerRoundingType cornerRoundingType;  // default=Slow CALayer .cornerRadius (offscreen rendering)
+
+/** @abstract The radius to use when rounding corners of the ASDisplayNode.
+ *
+ * @discussion This property is thread-safe and should always be preferred over CALayer's cornerRadius property,
+ * even if corner rounding type is ASCornerRoundingTypeDefaultSlowCALayer.
+ */
 @property (nonatomic, assign)           CGFloat cornerRadius;                     // default=0.0
 
 @property (nonatomic, assign)           BOOL clipsToBounds;                    // default==NO