update ns_config_ets_dup atomically

I.e. so that changes made as one (i.e. via cas_config) are not seen partially.

Change-Id: I1cb45859f2d55d2ec126c94e021d21589e024130
Reviewed-on: http://review.couchbase.org/44628
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>