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/Items/Bulletin Boards/BulletinMessage.cs

@@ -12,7 +12,7 @@ namespace Server.Items
 
             Poster = poster;
             Subject = subject;
-            Time = DateTime.UtcNow;
+            Time = Core.Now;
             LastPostTime = Time;
             Thread = thread;
             PostedName = Poster.Name;