Write a file `uuid` in server's data directory that contains random UUID, generated at startup if file does not exist.

It will be used for several purposes:
- to anonymously identify server in Sentry (currently Sentry is unusable);
- (possible future usage) to identify server in ensemble.

@tavplubix comments?
createServer(listen_host, port_name, listen_try, [&](UInt16 port)
            {
                Poco::Net::ServerSocket socket;
                auto address = socketBindListen(socket, listen_host, port,  true);
                socket.setReceiveTimeout(Poco::Timespan());
                socket.setSendTimeout(settings.send_timeout);
                servers->emplace_back(port_name, std::make_unique<Poco::Net::TCPServer>(
                    new MySQLHandlerFactory(*this),
                    server_pool,
                    socket,
                    new Poco::Net::TCPServerParams));
                LOG_INFO(log, "Listening for MySQL compatibility protocol: {}", address.toString());
            });
            port_name = "postgresql_port";
            createServer(listen_host, port_name, listen_try, [&](UInt16 port)
            {
                Poco::Net::ServerSocket socket;
                auto address = socketBindListen(socket, listen_host, port,  true);
                socket.setReceiveTimeout(Poco::Timespan());
                socket.setSendTimeout(settings.send_timeout);
                servers->emplace_back(port_name, std::make_unique<Poco::Net::TCPServer>(
                    new PostgreSQLHandlerFactory(*this),
                    server_pool,
                    socket,
                    new Poco::Net::TCPServerParams));
                LOG_INFO(log, "Listening for PostgreSQL compatibility protocol: " + address.toString());
            });
#if USE_GRPC
            port_name = "grpc_port";
            createServer(listen_host, port_name, listen_try, [&](UInt16 port)