state: refused writes limited for the node too

Review 3, R5: --refused-writes-per-minute bounds each user, but the
cost it guards, a document copy under the write lock, is the node's:
ten users, or ten accounts of one, are ten times the budget.

The node now takes six times a user's limit within a minute in all
(refused::NODE_FACTOR); past it, every user with a write refused
within the minute is answered 429 untried, until the oldest leave it.
A user none of whose writes was refused writes on, so honest writers
aren't held up by others' refusals.

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