Wesnothd Connection: used a void promise at celmin's suggestion

src/wesnothd_connection.cpp

@@ -166,7 +166,7 @@ void wesnothd_connection::handle_handshake(const error_code& ec)
 		throw system_error(ec);
 	}
 
-	handshake_finished_.set_value(true);
+	handshake_finished_.set_value();
 	recv();
 }
 

src/wesnothd_connection.hpp

@@ -134,7 +134,7 @@ private:
 
 	std::mutex last_error_mutex_;
 
-	std::promise<bool> handshake_finished_;
+	std::promise<void> handshake_finished_;
 
 	boost::asio::streambuf read_buf_;