CBQE-0: increase batch size in warmuptest

Change-Id: I763a98c3cc37d22b53e0ed5881364d08501054b7
Reviewed-on: http://review.couchbase.org/34889
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>