Add optimistic concurrency to batch processing

[JackLewis-digirati]

Requirement originated from code review comment related to "batch completed" notification processing to generate manifests: #214 (comment)

Original comment:

Are there any potential issues with this if we scaled up? E.g. if there was a Manifest that had 3 batches and we had scaled to 3 engines and 3 "complete" listeners. If each of those listeners received a "complete" notification at the same time for each of the 3 batches - is there a chance something could fall through, and the 3 batches are separately marked as 'complete' at the same time so the final "done" logic isn't fired?

I wonder if we could add a optimistic concurrency token to Batch to handle that? ie one of the 3 batches would fail and be requeued (not for this ticket, can revisit)

Ultimately the requirement is to prevent concurrent competing notification all marking separate batches as 'done' but no 1 single processor running the "batch completion" logic, meaning the Manifest wouldn't be generated.

An optimistic concurrency token was suggested as EF has native support for this: https://www.npgsql.org/efcore/modeling/concurrency.html?tabs=fluent-api and it would be a relatively rare occurrence.