changes all instances accessing cache by key "contentItem" to use CacheKeys.ContentItemCache and replaces

all of it's cache usages with the ApplicationContext.Current.ApplicationCache. Obsoletes a couple of methods
that are not used.

src/Umbraco.Core/Cache/NullCacheProvider.cs

@@ -66,6 +66,10 @@ namespace Umbraco.Core.Cache
             return getCacheItem();
         }
 
+        public override void InsertCacheItem<T>(string cacheKey, CacheItemPriority priority, Func<T> getCacheItem)
+        {
+        }
+
         public override void InsertCacheItem<T>(string cacheKey, CacheItemPriority priority, TimeSpan timeout, Func<T> getCacheItem)
         {
         }