update comments regarding converting shared_ptr to unique_ptr

include/libtorrent/aux_/session_impl.hpp

@@ -166,7 +166,8 @@ namespace libtorrent
 		// the actual sockets (TCP listen socket and UDP socket)
 		// An entry does not necessarily have a UDP or TCP socket. One of these
 		// pointers may be null!
-		// TODO: 3 make these unique_ptr<>
+		// These must be shared_ptr to avoid a dangling reference if an
+		// incoming packet is in the event queue when the socket is erased
 		boost::shared_ptr<tcp::acceptor> sock;
 		boost::shared_ptr<udp_socket> udp_sock;
 	};

include/libtorrent/tracker_manager.hpp

@@ -401,7 +401,8 @@ namespace libtorrent
 		mutable std::mutex m_mutex;
 
 		// maps transactionid to the udp_tracker_connection
-		// TODO: this should be unique_ptr in the future
+		// These must use shared_ptr to avoid a dangling reference
+		// if a connection is erased while a timeout event is in the queue
 		typedef boost::unordered_map<boost::uint32_t
 			, boost::shared_ptr<udp_tracker_connection> > udp_conns_t;
 		udp_conns_t m_udp_conns;