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/Misc/PoolOfAcid.cs

@@ -27,7 +27,7 @@ namespace Server.Items
 
             m_MinDamage = minDamage;
             m_MaxDamage = maxDamage;
-            m_Created = DateTime.UtcNow;
+            m_Created = Core.Now;
             m_Duration = duration;
 
             m_Timer = Timer.DelayCall(TimeSpan.Zero, TimeSpan.FromSeconds(1), OnTick);
@@ -46,7 +46,7 @@ namespace Server.Items
 
         private void OnTick()
         {
-            var now = DateTime.UtcNow;
+            var now = Core.Now;
             var age = now - m_Created;
 
             if (age > m_Duration)