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/Multis/Houses/HouseSign.cs

@@ -146,7 +146,7 @@ namespace Server.Multis
                 if (canClaim && !BaseHouse.HasAccountHouse(from))
                 {
                     Owner.Owner = from;
-                    Owner.LastTraded = DateTime.UtcNow;
+                    Owner.LastTraded = Core.Now;
                 }
             }