Cancel a request; its link then answers 410 REQUEST_CLOSED (default workspace only)

Allowed over a half-done accept (the escape hatch): the Net Zero rows it wrote stay as ordinary evidence, and the response's leftoverNetZeroRows counts them. 409 codes: INVALID_STATE (already accepted or cancelled), REVIEW_IN_PROGRESS (another accept, return or cancel of this request is running), WORKSPACE_SCOPE_UNSUPPORTED (outside the default workspace).