SourceNode uses too much memory because of AnnotationList

[ewoutkramer]

Describe the bug
Much like #603 , SourceNode creates a new instance of a Lazy<AnnotationList> for each and every node in the SourceNode. Which can be many when dealing with big resources.

Expected behavior
We should not allocate a fresh instance of Lazy<T> but use the static Lazy.EnsureInitialized instead:

In DomNode.cs:

    private readonly Lazy<AnnotationList> _annotations = new Lazy<AnnotationList>(() => new AnnotationList());
    protected AnnotationList AnnotationsInternal { get { return _annotations.Value; } }

should be changed to:

     private AnnotationList _annotations = null;
     private AnnotationList AnnotationsInternal => LazyInitializer.EnsureInitialized(ref _annotations);

This might be worth doing with the ChildList property too (in the same file):

   protected List<T> ChildList = new List<T>();

Additional context
Reported by the FirelyServer team, marked as Firely Favorite.