Fix #602 Turn on socket keepalive

contrib/epee/include/net/abstract_tcp_server2.inl

@@ -935,6 +935,9 @@ POP_WARNINGS
         boost::bind(&boosted_tcp_server<t_protocol_handler>::handle_accept, this,
         boost::asio::placeholders::error));
 
+      boost::asio::socket_base::keep_alive opt(true);
+      conn->socket().set_option(opt);
+
       conn->start(true, 1 < m_threads_count);
       conn->save_dbg_log();
     }else