use plain debug logger in concurrency_throttle

Change-Id: I207d5dccaea5a92e32aa44027f026686392c2cc8
Reviewed-on: http://review.couchbase.org/37817
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>