optimize histogram for fixed-width bins
optimize histogram for fixed-width bins

Change-Id: I0c8c71537def4b1f361bd98c93203d6ea4193aeb
Reviewed-on: http://review.northscale.com/2103
Reviewed-by: Steve Yen <ef850f73f7832052d3c6d9b5ddd3a1716d9a57f1@gmail.com>
Tested-by: Steve Yen <ef850f73f7832052d3c6d9b5ddd3a1716d9a57f1@gmail.com>

zbase/moxi,zbase/moxi,couchbase/moxi,membase/moxi,steveyen/moxi,membase/moxi,steveyen/moxi,membase/moxi,zbase/moxi,steveyen/moxi,couchbase/moxi,zbase/moxi,couchbase/moxi,membase/moxi,couchbase/moxi,steveyen/moxi,couchbase/moxi,couchbase/moxi,membase/moxi,steveyen/moxi,membase/moxi,zbase/moxi