Core/Groups: Use std::chrono type for Group::m_readyCheckTimer

Port From (https://github.com/TrinityCore/TrinityCore/commit/48b63c4c6721fa9a38f74f8fa92255ac0938da42)

Source/Game/Handlers/GroupHandler.cs

@@ -20,6 +20,7 @@ using Game.Entities;
 using Game.Groups;
 using Game.Networking;
 using Game.Networking.Packets;
+using System;
 
 namespace Game
 {
@@ -569,7 +570,7 @@ namespace Game
                 return;
 
             // everything's fine, do it
-            group.StartReadyCheck(GetPlayer().GetGUID(), packet.PartyIndex);
+            group.StartReadyCheck(GetPlayer().GetGUID(), packet.PartyIndex, TimeSpan.FromMilliseconds(MapConst.ReadycheckDuration));
         }
 
         [WorldPacketHandler(ClientOpcodes.ReadyCheckResponse, Processing = PacketProcessing.Inplace)]