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/Engines/Quests/Core/QuestSystem.cs

@@ -473,7 +473,7 @@ namespace Server.Engines.Quests
                     {
                         var endTime = restartInfo.RestartTime;
 
-                        if (DateTime.UtcNow < endTime)
+                        if (Core.Now < endTime)
                         {
                             inRestartPeriod = true;
                             return false;