The coming issues module hands out issue numbers from its own state; two instances must never hand out one
number twice. Create and Update already write by compare-and-set (ADR 0259), so State(...).Increment reads,
writes one more at the revision read, and on a lost write reads and tries again, bounded by the caller's
attempts. A lost write past the bound is ErrChanged, any other refusal and a value that is no counter are
returned at once: nothing is reported counted that was not written.
Part of the issue tracker decision, hq research 042.
