Fixes: #U4-1986 - data type cache is not refreshed in load balanced environments.

src/Umbraco.Core/Cache/CacheKeys.cs

@@ -35,5 +35,7 @@ namespace Umbraco.Core.Cache
         public const string StylesheetCacheKey = "UmbracoStylesheet";
         public const string StylesheetPropertyCacheKey = "UmbracoStylesheetProperty";
 
+        public const string DataTypeCacheKey = "UmbracoDataTypeDefinition";
+
     }
 }