Poll a job — runs the next slice if one is due, then answers

Scope: solve. If the job is still running and no other request is working on it, this request runs about 3 s more of trials (allow up to 5 s) before answering, so a job progresses exactly as fast as it is polled (meta.advanced says whether this poll worked). When job.status is done the body carries result; failed carries job.error; cancelled and expired carry neither. A job the key did not create, or one older than 24 hours, is 404.