Unique jobs

Unique{Key} makes Enqueue return the id of a live job with the same kind and key instead of inserting another one. The key is released as soon as that job succeeds, fails or is deleted.

Unique{Key, For: d} holds the key for d from the first enqueue, whatever happens to the job in the meantime, including success. It means "at most once per d" (one reminder per 10 minutes), not "no duplicates while it runs".

Unique{Key, Replace: true} updates a holder that hasn't started yet with the new args, meta, tags, title and priority, so the job runs with the latest data. Unique{Key, Debounce: d} runs the job d after the last enqueue: every new enqueue pushes a holder that is still waiting and replaces its args, which suits work like "reindex once the user stops editing". A holder that is already running is never touched.