Converts the media cache refresher to be a json cache refresher since it was impossible before to have media cache

cleared when media was deleted. Created base classes for cache refreshers, we now have a new event - CacheUpdated
which can now be used by code to execute on each individual server when any cache refresher is updated. Listening to events
normally only fire on the individual server so if people are wanting to refresh their own cache there was previously no way
to do that.

src/Umbraco.Core/Models/Template.cs

@@ -29,7 +29,7 @@ namespace Umbraco.Core.Models
         private static readonly PropertyInfo SortOrderSelector = ExpressionHelper.GetPropertyInfo<Template, int>(x => x.SortOrder);
         private static readonly PropertyInfo ParentIdSelector = ExpressionHelper.GetPropertyInfo<Template, int>(x => x.ParentId);
         private static readonly PropertyInfo NodePathSelector = ExpressionHelper.GetPropertyInfo<Template, string>(x => x.NodePath);
-        private static readonly PropertyInfo MasterTemplateIdSelector = ExpressionHelper.GetPropertyInfo<Template, int>(x => x.MasterTemplateId);
+        //private static readonly PropertyInfo MasterTemplateIdSelector = ExpressionHelper.GetPropertyInfo<Template, int>(x => x.MasterTemplateId);
         private static readonly PropertyInfo MasterTemplateAliasSelector = ExpressionHelper.GetPropertyInfo<Template, string>(x => x.MasterTemplateAlias);