buf_flush_remove(): Disable the output for now, because we
certainly do not want this after every page flush on shutdown.
It must be rate-limited somehow. There already is a timeout
extension for waiting the page cleaner to exit in
logs_empty_and_mark_files_at_shutdown().

log_write_up_to(): Use correct format.

srv_purge_should_exit(): Move the timeout extension to the
appropriate place, from one of the callers.