Should POST to datapoint (no upserts) throw errors on concurrent modification

[s4ke]

Currently, the last POST silently wins when two start at a time where no entry exists because we are simply using the standard upsert logic. While this is good for most cases, this might be surprising in some situations.

Instead, we could simply remove the ON CONFLICT logic when doing a regular POST.

[s4ke]

We can handle such a race condition that was not handled by the validation logic by returning a HTTP 409 conflict.