ship the new updater

Summary:
It's time to ship the new updater! `IGListExperimentalAdapterUpdater` has been running on Instagram for a couple months with better performance and stability. In this diff, we're renaming `IGListExperimentalAdapterUpdater` (and related classes) to `IGListAdapterUpdater`.

Here's a recap:

## Stability

* `[IGListAdapter setDataSource]` isn't safe
  * We're changing the underlying data without telling the `UICollectionView`.
  * Fix: Lets invalidate the `UICollectionView` data by changing its dataSource.
* `[IGListAdapter setCollectionView]` isn't safe
  * This is synchronous, but we might have pending or on-going updates. The `UICollectionView` might get synced before the pending update actually start executing, so the diff results will be off.
  * Fix: Lets wrap updates in a transaction that can be cancelled.
* Returning a nil `IGListSectionController` from `IGListAdapterDataSource` could crash
  * The `IGListAdapterUpdater` will still perform the diffing assuming that all the objects will have a section, which isn't the case.
  * Fix: Lets generate the `IGListSectionController` before the diffing.
* Other improvements
  * Lets ask for the `fromObject` just before diffing, instead of asking when scheduling the update.
  * If the `UICollectionView` section count doesn't match `fromObject`, lets fallback to a reload.

## Performance

* Re-test background diffing
  * `IGListExperimentBackgroundDiffing` and coalescing updates wasn't safe because of the issues mentioned above. The longer we wait, the more likely we'll end up in a race condition. Lets try re-testing with the stability improvements.
* Unblocks background layout calculation
  * This is a larger project, but these improvements are required to make background work safe.
* Only create the `backgroundView` if needed (although this doesn't really require the new updater)

## Other

* Transactions
  * `IGListAdapterUpdater` is the workhorse of `IGListKit` and has become a bit hard to follow over the years. We want to break it apart into simpler, more manageable parts.
* Avoid blocks
  * There's a lot of blocks flying around, making crash logs hard to read. Lets try to use methods/functions where possible.

Reviewed By: Haud, lorixx

Differential Revision: D25884782

fbshipit-source-id: 1357fa23513a239051d5b1766823effa3199f656

Source/IGListDiffKit/IGListExperiments.h

@@ -20,7 +20,7 @@ typedef NS_OPTIONS (NSInteger, IGListExperiment) {
     IGListExperimentBackgroundDiffing = 1 << 2,
     /// Test invalidating layout when cell reloads/updates in IGListBindingSectionController.
     IGListExperimentInvalidateLayoutForUpdates = 1 << 3,
-    /// Test skipping performBatchUpdate if we don't have any updates. `IGListExperimentalAdapterUpdater` only.
+    /// Test skipping performBatchUpdate if we don't have any updates.
     IGListExperimentSkipPerformUpdateIfPossible = 1 << 4,
     /// Test skipping creating {view : section controller} map, which has inconsistency issue.
     IGListExperimentSkipViewSectionControllerMap = 1 << 5

Source/IGListKit/IGListAdapterUpdater.h

@@ -9,6 +9,7 @@
 
 #import <IGListDiffKit/IGListMacros.h>
 #import <IGListKit/IGListAdapterUpdaterCompatible.h>
+#import <IGListKit/IGListUpdatingDelegateExperimental.h>
 
 NS_ASSUME_NONNULL_BEGIN
 
@@ -22,7 +23,7 @@ NS_ASSUME_NONNULL_BEGIN
  */
 IGLK_SUBCLASSING_RESTRICTED
 NS_SWIFT_NAME(ListAdapterUpdater)
-@interface IGListAdapterUpdater : NSObject <IGListAdapterUpdaterCompatible>
+@interface IGListAdapterUpdater : NSObject <IGListAdapterUpdaterCompatible, IGListUpdatingDelegateExperimental>
 
 @end