Make PersistenceState byte enum

Author: SergeiPavlov

Orm/Xtensive.Orm/Orm/PersistenceState.cs

@@ -9,7 +9,7 @@ namespace Xtensive.Orm
   /// <summary>
   /// Defines possible persistence states of the entities.
   /// </summary>
-  public enum PersistenceState
+  public enum PersistenceState : byte
   {
     /// <summary>
     /// The entity is synchronized with the database (there are no unsaved changes).
@@ -28,4 +28,4 @@ public enum PersistenceState
     /// </summary>
     Removed = 3,
   }
-}
+}