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/Targeting/SpellTargetPoint3D.cs

@@ -40,7 +40,7 @@ namespace Server.Spells
 
             from.SendLocalizedMessage(501943); // Target cannot be seen. Try again.
             from.Target = new SpellTargetPoint3D(m_Spell);
-            from.Target.BeginTimeout(from, TimeoutTime - DateTime.UtcNow);
+            from.Target.BeginTimeout(from, TimeoutTime - Core.TickCount);
             m_Spell = null; // Needed?
         }