ZoneOnDemand.AddAsync().

[jtkech]

The overlaoded Shape.Add() in ZoneOnDemand do the following

dynamic zone = _zoneFactory().GetAwaiter().GetResult();

So here i added and used an async version that do

dynamic zone = await _zoneFactory();

Maybe related to #3860 where it seems that the dyn layout is not well built, e.g without a Zones prop.

[jtkech]

@sebastienros

Maybe this one is interesting for perf because here, when we add a zone, we prevent its factory, that may do session queries, to run synchronously, e.g through the layer filter.

If it will be merged, i will integrate it in 3.0 for benchmark testing.