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/BufferReader.cs

@@ -37,6 +37,8 @@ namespace Server
             _encoding = encoding ?? TextEncoding.UTF8;
         }
 
+        public BufferReader(byte[] buffer, DateTime LastSerialized) : this(buffer) => LastSerialized = LastSerialized;
+
         public void Reset(byte[] newBuffer, out byte[] oldBuffer)
         {
             oldBuffer = _buffer;
@@ -45,6 +47,8 @@ namespace Server
         }
 
         // Compatible with BinaryReader.ReadString()
+        public DateTime LastSerialized { get; init; } = DateTime.MinValue;
+
         public string ReadString(bool intern = false)
         {
             if (!ReadBool())