MB-100: added ability to search_xxx for latest config

We can see tons of places where we're doing something like:

ns_config:search_prop(ns_config:get(), xxx, yyy)

Given we're fine with latest config in that places it makes sense to
use eval-based search instead of getting full config and searching it.

So now we have special way to indicate latest config which is atom
'latest-config-marker'. So code above becomes:

ns_config:search_prop('latest-config-marker', xxx, yyy)

and is measurably faster

Change-Id: Iadac2ad13504b63ece0496d4d2a224168fec8d15
Reviewed-on: http://review.couchbase.org/27579
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>