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/Handlers/QuestHandler.cpp

@@ -627,7 +627,7 @@ void WorldSession::HandleQuestgiverStatusMultipleQuery(WorldPacket& /*recvPacket
     WorldPacket data(SMSG_QUESTGIVER_STATUS_MULTIPLE, 4);
     data << uint32(count);                                  // placeholder
 
-    for (GuidSet::const_iterator itr = _player->m_clientGUIDs.begin(); itr != _player->m_clientGUIDs.end(); ++itr)
+    for (auto itr = _player->m_clientGUIDs.begin(); itr != _player->m_clientGUIDs.end(); ++itr)
     {
         uint32 questStatus = DIALOG_STATUS_NONE;