fix leakage of ServerGroupChatRoom, resulting in the conference server using 100% cpu for several seconds just to find list subscription handlers.
replace unefficient use of std::list managing list subscriptions by std::unordered_map
fix very costly == operator of ConferenceId and IdentityAddress, as they were instanciating a Address (wrapping belle_sip_header_address_t) in toString(). This change should greatly improve the chat rooms load time.
