state: limit refused writes per user

A write every node would refuse is rolled back by forking the document
at the heads before it, under the write lock, at a cost that grows with
the document's history. A user could ask for such writes in a loop.

Count each user's refused writes over the last minute. Past
--refused-writes-per-minute (default 10), its writes are answered 429
without being tried, until the oldest refusal leaves the minute.

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