epee: regularly cleanup connections we kept a reference to

Since connections from the ::connect method are now kept in
a deque to be able to cancel them on exit, this leaks both
memory and a file descriptor. Here, we clean those up after
30 seconds, to avoid this. 30 seconds is higher then the
5 second timeout used in the async code, so this should be
safe. However, this is an assumption which would break if
that async code was to start relying on longer timeouts.

contrib/epee/include/net/abstract_tcp_server2.h

@@ -279,6 +279,8 @@ 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_;    
@@ -306,7 +308,7 @@ namespace net_utils
     connection_ptr new_connection_;
 
     std::mutex connections_mutex;
-    std::deque<connection_ptr> connections_;
+    std::deque<std::pair<boost::system_time, connection_ptr>> connections_;
 
   }; // class <>boosted_tcp_server