added ns_config_default:latest_config_marker()

To have dialyzer-verified way of getting latest-config-marker.

Change-Id: Ibed27dca669abf48e9418e79ee2fcb060ba0aa7a
Reviewed-on: http://review.couchbase.org/42008
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>