abstract_tcp_server2: improve tracking/cancelling of early connections

We don't actually need to keep them past the call to start, as this
adds them to the config object list, and so they'll then be cancelled
already when the stop signal arrives. This allows removing the periodic
call to cleanup connections.

contrib/epee/include/net/abstract_tcp_server2.h

@@ -281,8 +281,6 @@ namespace net_utils
 
     bool is_thread_worker();
 
-    bool cleanup_connections();
-
     /// The io_service used to perform asynchronous operations.
     std::unique_ptr<boost::asio::io_service> m_io_service_local_instance;
     boost::asio::io_service& io_service_;    
@@ -309,7 +307,7 @@ namespace net_utils
     connection_ptr new_connection_;
 
     boost::mutex connections_mutex;
-    std::deque<std::pair<boost::system_time, connection_ptr>> connections_;
+    std::set<connection_ptr> connections_;
 
   }; // class <>boosted_tcp_server