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/Spells/Ninjitsu/MirrorImage.cs

@@ -171,7 +171,7 @@ namespace Server.Mobiles
             var duration = TimeSpan.FromSeconds(30 + caster.Skills.Ninjitsu.Fixed / 40);
 
             new UnsummonTimer(caster, this, duration).Start();
-            SummonEnd = DateTime.UtcNow + duration;
+            SummonEnd = Core.Now + duration;
 
             MirrorImage.AddClone(m_Caster);
         }