The primary motivation for this change is to support databases that
don't take a path, e.g. out of process databases.
This configuration structure leaves the door open for other media
storage mechanisms in the future, such as S3.
It's also structured to avoid `#[serde(flatten)]` so that we can use
`#[serde(deny_unknown_fields)]`.
This will break backwards compatibility of configurations, but
ensures that a previously-configured setting won't get dropped
arbitrarily. Pretty much worth it, I think.
