logwriter: throttle bug fixed

in log_writer_perform() we should use LW_FLUSH_NORMAL, otherwise
throttle won't work

flush_waiting_for_timeout renamed to waiting_for_throttle
- log_queue_check_items() set timeout_msec if throttle is used

Signed-off-by: Budai Laszlo <Laszlo.Budai@balabit.com>