Make partial submission progress survive retries

[lognaturel]

Submissions where some amount of the upload succeeds, and the rest doesn't, retrying should not require every chunk to be uploaded again.

Notes

From @FormShareDev at #7353:

We used ODK Collect a couple of years ago in rural Tanzania as part of the ARTEMIS project, where we collected farm data along with close to 100 high-res images. Sending such a submission is already hard there, and today's upload behavior makes it harder: there is no chunk-level progress, so whenever an upload fails, which happens often on an unreliable link, the entire submission is re-uploaded from the beginning on the next attempt.

Possible solution at #7353 using a dot file to store chunk submission status.

[lognaturel]

One related thing that we've discussed in the past is that the X-OpenRosa-Accept-Content-Length conflates maximum request size and preferred batching size. The proposed approach at #7353 which relies on X-OpenRosa-Accept-Content-Length-based chunking wouldn't help most ODK Central users with the default configuration, for example, because the X-OpenRosa-Accept-Content-Length is currently set to 100mb. We set this default because we've heard more need for single large attachments than many small ones. We've seen some workflows with many images on unreliable connections but background autosend generally seems to get the single big chunk through eventually even if it's not ideal.

As far as I know, the main reason for chunking rather than sending attachments one by one is not to have to send the submission XML too many redundant times. The submission protocol requires every POST to come with submission XML.