Scrolling loses inertia when rendering more items from data source?

## New issue checklist

- [x] I have reviewed the [`README`](https://github.com/Instagram/IGListKit/blob/master/README.md) and [documentation](http://instagram.github.io/IGListKit)
- [x] I have searched [existing issues](https://github.com/Instagram/IGListKit/issues) and this is not a duplicate

### General information

- `IGListKit` version:
- iOS version(s): 11.2
- CocoaPods/Carthage version: 3.2.0
- Xcode version: 9.2
- Devices/Simulators affected: All that I tested with: Phone 6 (device), iPhone X (device), iPhone SE (simulator)
- Reproducible in the demo project? (Yes/No): https://github.com/achan/hesijimbo-ios
- Related issues: Possibly similar to https://github.com/Instagram/IGListKit/issues/1004

### Debug information

```bash
# Please include debug logs using the following lldb command:
po [IGListDebugger dump]
```

I'm still new to IGListKit so forgive me if I have this all wrong.

In two separate apps, one developed by me and another developed by a co-worker, we've seen scrolling inertia abruptly stop when trying to load more items to our `UICollectionView` (a la infinite scroll).

They're both implemented similarly to [this](https://github.com/achan/hesijimbo-ios/blob/ae162d96fed545a6c2cef024b20c8d43027f157f/HesiJimbo/Video%20Feed/VideoFeedController.swift#L61-L66):

```swift
// VideoFeedController.swift
override func scrollViewWillEndDragging(_ scrollView: UIScrollView, withVelocity velocity: CGPoint, targetContentOffset: UnsafeMutablePointer) {
let distance = scrollView.contentSize.height - (targetContentOffset.pointee.y + scrollView.bounds.height)
if !viewModel.isLoading && distance < 200 {
loadMore()
}
}
```

Where `loadMore()` is ultimately performing a network request, converting the request into `ListDiffable` view models, appending to the data source and telling the `adapter` to perform updates.

[`UICollectionViewController`](https://github.com/achan/hesijimbo-ios/blob/ae162d96fed545a6c2cef024b20c8d43027f157f/HesiJimbo/Video%20Feed/VideoFeedController.swift#L76-L86):
```swift
private func load(request: Promise) {
adapter.performUpdates(animated: true, completion: nil)
_ = request.done { [weak self] listing in
guard let strongSelf = self else {
return
}

strongSelf.adapter.performUpdates(animated: false, completion: nil)
strongSelf.refresher.endRefreshing()
}
}
```

[`View Model`](https://github.com/achan/hesijimbo-ios/blob/master/HesiJimbo/Video%20Feed/VideoFeedViewModel.swift#L24-L36):
```swift
@discardableResult
func loadMore() -> Promise {
isLoading = true
return service.perform(pagination: pagination).done { [weak self] listing in
self?.loaded(listing: listing)
}
}

private func loaded(listing: VideoFeedListing) {
isLoading = false
items.append(contentsOf: listing.items)
pagination = listing.pagination
}
```

I've played around with moving loadMore() to a background thread with similar outcome, though I believe network requests from URLSessionDataTask do that automatically?

Here's a screen capture of the bug in action. I've added a print statement for emphasis.


Thanks for your time and all the hard work on this library 👏

Reproduce the scrolling behavior in the linked HesiJimbo demo, starting at VideoFeedController.swift and its scrollViewWillEndDragging and load methods. Trace the interaction between loadMore(), the view model's item append, and adapter.performUpdates; compare with related issue #1004. Done means infinite scrolling adds items without abruptly stopping UICollectionView scrolling inertia.

Written by the indexing model from the issue text.