MB-100: using dict:update_counter to update load stats table

Change-Id: Ibdc29ef246b471dfcdb29b7c4720df4c2c568fb5
Reviewed-on: http://review.couchbase.org/27090
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Junyi Xie <junyi.couchbase@gmail.com>