* - [ASDisplayNode] Consolidate main thread initialization and allow apps to invoke it manually instead of +load.
Additionally this has a few minor fixes for Yoga support, and adds some basic
but universally valuable callbacks like -nodeDidLoad to ASNodeController.
* Small fix for handling _layoutVersion.
* Remove poking the scale accessor
* Make our async deallocation functions take a double pointer, so we can be sure we've released before the queue drains
* Make it a class property
* Fix the return type
* Use a locker
* Improve release notes
* Implement tests for the layout flattening process
* Refactor the flattening algorithm
- Remove flattened flag
- No more self check
- Stop traversing a layout tree branch when hits a displaynode node.
- Reuse as many existing ASLayout objects as possible
* Update changelog
* Ceil position values before comparing
* Explain why sublayout elements must be retained
