Core/Player: Optimize Player::HaveAtClient() performance

Change m_clientGUIDs from std::set to std::unordered_set to reduce by 2.7x times the cpu usage in Player::HaveAtClient()

src/server/game/Grids/Notifiers/GridNotifiers.cpp

@@ -65,7 +65,7 @@ void VisibleNotifier::SendToSelf()
         }
     }
 
-    for (GuidSet::const_iterator it = vis_guids.begin(); it != vis_guids.end(); ++it)
+    for (auto it = vis_guids.begin(); it != vis_guids.end(); ++it)
     {
         i_player.m_clientGUIDs.erase(*it);
         i_data.AddOutOfRangeGUID(*it);