MB-10400: added ns_server_testrunner_api:shutdown_nicely/0

I.e. for testing clean shutdown of couchbase

Change-Id: I6830af486880a823be8ac6b958a9e03bb7b53645
Reviewed-on: http://review.couchbase.org/34807
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
MB-10400: don't import a bunch of couch_util functions in header file

Change-Id: I2472efedca9c0789d4648678a1260901707928ed
Reviewed-on: http://review.couchbase.org/34808
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>