reindent concurrency_throttle

Change-Id: I35d4331186dbddd87fe340cfae7ddf9e0fbb2e90
Reviewed-on: http://review.couchbase.org/37816
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>