Creating or publishing form draft is not atomic

@lognaturel was working on a server with a large number of form versions. Quoting @lognaturel from Slack:

> Creating a form draft takes a very long time and sometimes 504s. This is for any form on the server. Publishing a form draft generally 504s. Sometimes it succeeds but CSV attachments are not successfully added.

> The behavior I’m experiencing is exactly what @matthew-white described at getodk/central-backend#381.

> I do think publish should be atomic and it doesn’t look like it is. It’s a bad state to have tested a draft with an attachment and then to have it not be there when published.

> Both draft creation and draft publish [are not atomic].