[ASDisplayNode] Trigger a layout pass whenever a node enters preload state (#3263)

* Add a thread-safe layoutIfNeeded implementation to ASDisplayNode

* Trigger a layout pass when a display node enters preload state
- This ensures that all the subnodes have the correct size to preload their content.

* ASCollectionNode to trigger its initial data load when it enters preload state

* Minor change in _ASCollectionViewCell

* Layout sublayouts before dispatch to main for subclass hooks

* Update comments

* Don't wait until updates are committed when the collection node enters display state

* Same deal for table node

* Explain the layout trigger in ASDisplayNode

Source/ASDisplayNode.h

@@ -639,6 +639,11 @@ extern NSInteger const ASDefaultDrawingPriority;
  */
 - (void)setNeedsLayout;
 
+/**
+ * Performs a layout pass on the node. Convenience for use whether the view / layer is loaded or not. Safe to call from a background thread.
+ */
+- (void)layoutIfNeeded;
+
 @property (nonatomic, strong, nullable) id contents;                           // default=nil
 @property (nonatomic, assign)           BOOL clipsToBounds;                    // default==NO
 @property (nonatomic, getter=isOpaque)  BOOL opaque;                           // default==YES