Pass batch size to child processes

Rather than checking simplenews_threaded_send_throttle variable in simplenews_threaded_send_mail_spool(), pass it in using the $limit argument. This means that it also needs to be passed to the child process created in _simplenews_threaded_send_fsock(), but this will remove the need to temporarily set simplenews_threaded_send_throttle to 1 to send the test message on each SMTP server.