Avoid multiple initial data loads being issued by UICollectionView/UITableView (#3208)

* Avoid multiple initial data loads being issued by UICollectionView/UITableView
- Currently, there is a gap between the moment UICollectionView/UITableView triggers its first data load and when ASDataController finishes processing it. During this gap, the view keeps issuing "initial" loads by calling reloadData and causes its data controller to reload multiple times.
- Fix by immediately forward the first reloadData call to UICollectionView/UITableView before letting its data controller to handle it for real. During the gap, the view thinks that it loaded initial data but is empty, and thus stops triggering initial loads. Once the data controller finished loading, it will call another reloadData on the view which causes the view to swap to a correct state.

* Fix tests

* Use the existing flag of ASDataController

* Explain unit test

Source/ASCollectionView.mm

@@ -317,6 +317,12 @@ static NSString * const kReuseIdentifier = @"_ASCollectionReuseIdentifier";
 {
   ASDisplayNodeAssertMainThread();
   
+  if (! _dataController.initialReloadDataHasBeenCalled) {
+    // If this is the first reload, forward to super immediately to prevent it from triggering more "initial" loads while our data controller is working.
+    _superIsPendingDataLoad = YES;
+    [super reloadData];
+  }
+  
   void (^batchUpdatesCompletion)(BOOL);
   if (completion) {
     batchUpdatesCompletion = ^(BOOL) {