MB-100: short-cut node cookie access

And kill ns_cookie_manager:cookie_get()

Change-Id: I262ff2431458750474c1e186fcd32bb2b8742a24
Reviewed-on: http://review.couchbase.org/27671
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>