pillowfight: Properly generate key per num-items

Fixes a bug where we'd ignore the item limit if our own seqno was passed
(which is always).

Change-Id: Ie9666cd47a58a415a9c7c070ff4c6a4304343ba7
Reviewed-on: http://review.couchbase.org/38288
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>