nlbwmon: add init.d shutdown to preserve state on shutdown/reboot

State information since last commit interval is not saved during
shutdown/reboot.

By default the database directory is mounted on a tmpfs filesystem,
but becomes an issue when using a persistent directory.

Added to init.d stop sequence and included sleep to allow dying process
time to write state before filesystems are unmounted.

Signed-off-by: Chad Fraleigh <chadf@triularity.org>
Signed-off-by: Javier Marcet <javier@marcet.info>