shutdown niql service carefully by sending it shutdown message

Change-Id: I7f99a0296850b241e7939514ad43dc37f600a3b9
Reviewed-on: http://review.couchbase.org/43154
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>