[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/ASCollectionNode.mm

@@ -193,18 +193,20 @@
   [self.rangeController clearContents];
 }
 
-- (void)didExitPreloadState
-{
-  [super didExitPreloadState];
-  [self.rangeController clearPreloadedData];
-}
-
 - (void)interfaceStateDidChange:(ASInterfaceState)newState fromState:(ASInterfaceState)oldState
 {
   [super interfaceStateDidChange:newState fromState:oldState];
   [ASRangeController layoutDebugOverlayIfNeeded];
 }
 
+- (void)didEnterPreloadState
+{
+  // Intentionally allocate the view here so that super will trigger a layout pass on it which in turn will trigger the intial data load.
+  // We can get rid of this call later when ASDataController, ASRangeController and ASCollectionLayout can operate without the view.
+  [self view];
+  [super didEnterPreloadState];
+}
+
 #if ASRangeControllerLoggingEnabled
 - (void)didEnterVisibleState
 {
@@ -219,6 +221,12 @@
 }
 #endif
 
+- (void)didExitPreloadState
+{
+  [super didExitPreloadState];
+  [self.rangeController clearPreloadedData];
+}
+
 #pragma mark Setter / Getter
 
 // TODO: Implement this without the view. Then revisit ASLayoutElementCollectionTableSetTraitCollection