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/Help/SpeechLog.cs

@@ -66,7 +66,7 @@ namespace Server.Engines.Help
             {
                 var entry = m_Queue.Peek();
 
-                if (DateTime.UtcNow - entry.Created > EntryDuration)
+                if (Core.Now - entry.Created > EntryDuration)
                 {
                     m_Queue.Dequeue();
                 }
@@ -128,7 +128,7 @@ namespace Server.Engines.Help
         {
             From = from;
             Speech = speech;
-            Created = DateTime.UtcNow;
+            Created = Core.Now;
         }
 
         public Mobile From { get; }