This patch fixes the logic for counting items in batch operations.
Previously, the item count in requests was inaccurate, it count the
number of tabels in get_item and the request_items in write_items.
The new logic correctly counts each individual item in `BatchGetItem`
and `BatchWriteItem` requests.
Signed-off-by: Amnon Heiman <amnon@scylladb.com>
(cherry picked from commit 905408f764)