MB-100: speed up menelaus_auth:check_auth

I.e. via 'latest-config-marker' api.

Change-Id: I0bd2794e6b703035495115f6eaef4c7c93040481
Reviewed-on: http://review.couchbase.org/27581
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>