This provides internal features on _ASAsyncTransaction and ASDisplayNode to facilitate
implementing public API that allows clients to choose if they would prefer to block
on the completion of unfinished rendering, rather than allow a placeholder state to
become visible.
The internal features are:
-[_ASAsyncTransaction waitUntilComplete]
-[ASDisplayNode recursivelyEnsureDisplay]
Also provided are two such implementations:
-[ASCellNode setNeverShowPlaceholders:], which integrates with both Tables and Collections
-[ASViewController setNeverShowPlaceholders:], which should work with Nav and Tab controllers.
Lastly, on ASDisplayNode, a new property .shouldBypassEnsureDisplay allows individual node types
to exempt themselves from blocking the main thread on their display.
By implementing the feature at the ASCellNode level rather than ASTableView & ASCollectionView,
developers can retain fine-grained control on display characteristics. For example, certain
cell types may be appropriate to display to the user with placeholders, whereas others may not.
Follow-up work will include unit tests, revisiting names, and the header locations of definitions.
When setting a default placeholder and an attributedString before `_textKitComponents.textView` was created, the placeholder and the string were both appearing in the text node.
`_updateDisplayingPlaceholder` is called in `setAttributedString`, but since `_textKitComponents.textView` is nil the placeholder was not hidden. Calling `_updateDisplayingPlaceholder` after `_textKitComponents.textView` loads fixes this.
- Above is the generic case. Correctly handling it means relayout when the table view enters or leaves editing mode is solved as well.
- Async data source API removal: In a table view, cell nodes should always fill its content view and table view widths. Thus async data source can no longer provide custom constrained size for cell nodes. This removal allows table view to better handle relayout.
- Some more tests are added to ASTableViewTests to check against use cases handled in this diff.
