MB-8787 Use correct parameter in get_replication_batch_size().

worker_batch_size is the batch size in items not in kilobytes.

Change-Id: Id4b0a5ce24dc1de304ad5fe664cd0a0a93f63f59
Reviewed-on: http://review.couchbase.org/28161
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>