CBQE-672: add xdcr test for append with larger item size

Change-Id: I6ffa2c942dc37ec2e7ca58e81b89cf77f83b54d6
Reviewed-on: http://review.couchbase.org/29164
Reviewed-by: Ketaki Gangal <ketakigangal@gmail.com>
Tested-by: Iryna Mironava <irynamironava@yandex.ru>