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/UOContent/Special Systems/Engines/GiftGiving.cs

@@ -31,7 +31,7 @@ namespace Server.Misc
                 return;
             }
 
-            var now = DateTime.UtcNow;
+            var now = Core.Now;
 
             for (var i = 0; i < m_Givers.Count; ++i)
             {