use special code for merging config in ns_couchdb node

So that ns_couchdb does not attempt to bump changes counter.

Change-Id: If3e618423e1e258ed10065f7a2fe996c42aad64d
Reviewed-on: http://review.couchbase.org/44631
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>