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/Server/Serialization/BinaryFileReader.cs

@@ -30,6 +30,8 @@ namespace Server
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public void Close() => _reader.Close();
 
+        public DateTime LastSerialized { get; init; }
+
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public string ReadString(bool intern = false)
         {