Changes all instances of the legacy Cache class to be using ApplicationContext.Current.ApplicationCache.

Fixes: #U4-2039 - ensures macro xslt cache is invalidated in LB scenarios.

src/Umbraco.Core/CacheHelper.cs

@@ -273,6 +273,30 @@ namespace Umbraco.Core
             }
         }
 
+        /// <summary>
+        /// Gets (and adds if necessary) an item from the cache
+        /// </summary>
+        /// <typeparam name="TT"></typeparam>
+        /// <param name="cacheKey"></param>
+        /// <param name="priority"></param>
+        /// <param name="cacheDependency"></param>
+        /// <param name="getCacheItem"></param>
+        /// <returns></returns>
+        public TT GetCacheItem<TT>(string cacheKey,
+            CacheItemPriority priority,
+            CacheDependency cacheDependency,
+            Func<TT> getCacheItem)
+        {
+            if (!_enableCache)
+            {
+                return _nullCache.GetCacheItem<TT>(cacheKey, priority, null, cacheDependency, null, getCacheItem);
+            }
+            else
+            {
+                return _httpCache.GetCacheItem<TT>(cacheKey, priority, null, cacheDependency, null, getCacheItem);
+            }
+        }
+
         /// <summary>
         /// Inserts an item into the cache, if it already exists in the cache it will be replaced
         /// </summary>