Properly opt out of taskprov tasks

[mendess]

Discussion: #709 (comment)

Relevant spec: https://ietf-wg-ppm.github.io/draft-ietf-ppm-dap-taskprov/draft-ietf-ppm-dap-taskprov.html#section-4.4-3.1.1

We don't reject when a task id does not match the task config

[cjpatton]

To clarify: This only happens when we've already configured the task, in which case we use the previously configured task config to handle the request and not the advertised config.

[cjpatton]

Actually, it appears this is already enforced by parse_taskprov_advertisement(), which checks to make sure the task ID was computed properly from the advertised config. #742 adds a regression test for this.