loads batch within transaction for xml import

@jcoyne I tried a couple of things here, including putting a transaction around the entire update but that resulted in hitting `innodb_lock_wait_timeout` even when I increased the time out from 50 to 180 seconds.  Part of the issue is in doing a big batch of 50 images, the server cpu really gets pegged on dev (load avg around 7-8 on a dual cpu vm) and so the whole thing gets bogged down with the async processing of the images in the background too.  (I think this is in part why we see this problem more pervasively then you do without knowing the details of your AWS configuration) Because of the timeout, I tried to make the transaction as short as possible, and so I reload the batch record before updating the `uploaded_files`, this was the fastest configuration which worked with all the batches I've tried.