fix: Adds Created, LastSerialized, and BeforeSerialize for all entities. (#775)

* Adds `BeforeSerialized` for entities to handle cleanup.
* Adds `Created` and `LastSerialized` fields to all entities. While this is a big bloat, this will be necessary for identifying dangling references to other invalid entities.
* Changes formula for determining a valid reference to be _not null, not deleted, and reference's created date must be at or before the entities last serialized date_.
* Adds versioning to idx file and serializes `Created` and `LastSerialized`.
* Fixes Save Stats and also disables it by default.

Projects/UOContent/Misc/Guild.cs

@@ -1156,6 +1156,10 @@ namespace Server.Guilds
             list.TrimExcess();
         }
 
+        public override void BeforeSerialize()
+        {
+        }
+
         public override void Serialize(IGenericWriter writer)
         {
             if (LastFealty + TimeSpan.FromDays(1.0) < Core.Now)