Add FeatureReference back

Author: benaadams

src/Microsoft.AspNet.Http.Features/FeatureReference.cs

@@ -0,0 +1,38 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+namespace Microsoft.AspNet.Http.Features
+{
+    public struct FeatureReference<T>
+    {
+        private T _feature;
+        private int _revision;
+
+        private FeatureReference(T feature, int revision)
+        {
+            _feature = feature;
+            _revision = revision;
+        }
+
+        public static readonly FeatureReference<T> Default = new FeatureReference<T>(default(T), -1);
+
+        public T Fetch(IFeatureCollection features)
+        {
+            if (_revision == features.Revision)
+            {
+                return _feature;
+            }
+            _feature = (T)features[typeof(T)];
+            _revision = features.Revision;
+            return _feature;
+        }
+
+        public T Update(IFeatureCollection features, T feature)
+        {
+            features[typeof(T)] = feature;
+            _feature = feature;
+            _revision = features.Revision;
+            return feature;
+        }
+    }
+}