Make AsyncState thread safe

[mobratil]

AsyncState uses holder object AsyncStateHolder that uses List<object?> collection. Write operations to that collection are not thread safe including EnsureCapacity() method. If the async flow is shared between multiple threads the List<> and AsyncState is modified in parallel the list might lose some data.

The fix replaces List<object?> with custom Features class that encapsulates List<object?> collection and synchronizes when its size needs to be expanded.

Microsoft Reviewers: Open in CodeFlow