Improve audio normalization

* Move calculation of LUFS to a scheduled task as it's pretty slow
* Correctly calculate album LUFS
* Don't try to convert replaygain tags to LUFS values

MediaBrowser.Controller/Entities/BaseItem.cs

@@ -137,6 +137,13 @@ namespace MediaBrowser.Controller.Entities
         [JsonIgnore]
         public float? LUFS { get; set; }
 
+        /// <summary>
+        /// Gets or sets the gain required for audio normalization.
+        /// </summary>
+        /// <value>The gain required for audio normalization..</value>
+        [JsonIgnore]
+        public float? NormalizationGain { get; set; }
+
         /// <summary>
         /// Gets or sets the channel identifier.
         /// </summary>