fix(core): Caches DateTime.NowUtc (#548)

- [X] Caches DateTime.NowUtc on the game loop (not other threads)
- [X] Replaces all locations where it makes sense
- [X] Adds Min/Max for `IComparable` (TimeSpan, DateTimes, etc)

Closes #261

Projects/Server/Serialization/BufferWriter.cs

@@ -205,6 +205,8 @@ namespace Server
             Write(ticks);
         }
 
+        // TODO: Find a way to speed this up.
+        // Maybe used a special cached value?
         public void WriteDeltaTime(DateTime value)
         {
             var ticks = (value.Kind switch