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/IGenericReader.cs

@@ -21,6 +21,9 @@ namespace Server
 {
     public interface IGenericReader
     {
+        // Used to determine valid Entity deserialization
+        DateTime LastSerialized { get; init; }
+
         string ReadString(bool intern = false);
         long ReadLong();
         ulong ReadULong();