Had to remove the Resolution freezing idea because we are lazily instantiating the singleton instances, otherwise

we have to instantiate them all on startup which means that all type searching happens on startup and not lazily which
is bad for performance. I don't think its a big deal that we're not freezing these objects, MVC doesn't freeze its singletons
and nobody seems to have a problem with it, people will just know not to modify these items after startup.

src/Umbraco.Core/Resolving/SingleObjectResolverBase.cs

@@ -49,8 +49,6 @@ namespace Umbraco.Core.Resolving
 
 			set
 			{
-				Resolution.EnsureNotFrozen();
-
 				if (!_canBeNull && value == null)
 					throw new ArgumentNullException("value");
 				_resolved = value;