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.
