Keep a scan out of route-proxy's unrouted state: only names under a served domain, a few a minute, no visitor address (issue 774)

Review of ddae6729: a scanner's random Host or SNI names each became a key, bounded only by the
bucket's lifetime and size, and every failed write was retried at each change and logged each time.

A refused name is now kept by name only under the registrable domain of a name the proxy serves, and
at most five new names a minute; every other refusal is counted in memory and written once a minute
under <node>._other (last minute, since start), which route_proxy_unrouted shows. A refused write of
that state is dropped and logged once until a write succeeds; trimming memory to 4096 names moved
from the request to the writer. The visitor's address is no longer kept: the journal has it.

Quick: drop unrouted keys from strangers by a fixed allow-list. Thorough: bound by the proxy's own
domains, by rate, and by never retrying. Chose thorough: the scan is constant on a public machine.

modules/route-proxy/README.md

@@ -98,7 +98,7 @@ The proxy keeps two states, announced readable (`conn.Keeps`, novox/hq ADR 0495)
 | state | one key per | holds | written when |
 |---|---|---|---|
 | `routes` | served name | the rules it is served by (path, module, node, target, refusal, redirect, authentication); its certificate (authority, issuer, not-after, last renewal, outcome, or why it failed); the last upstream answer (status, time, error, path) | a route arrives, changes or leaves; an authority stores a certificate; a handshake fails to find one or hands out one not yet recorded; an upstream answer differs from the last one written, or the same one is a minute old |
-| `unrouted` | name answered "no route for", in a request or a TLS handshake | when it was last asked, by whom, how often since the start, whether it is an internal-only name asked from outside | first asked, then at most once a minute; deleted when the name is served; kept a week |
+| `unrouted` | name answered "no route for", in a request or a TLS handshake, under a domain the proxy serves | when it was last asked, how often since the start, whether it is an internal-only name asked from outside; never who asked (the journal keeps that, for fail2ban) | first asked, at most five new names a minute, then at most once a minute; deleted when the name is served; kept a week. Every other refusal is only counted, in one key per proxy (`<node>._other`: the last minute's count and the total), written once a minute. A refused write is dropped, never retried |
 
 Nothing is written on a timer. The provider helper's binary serves the module's tools from those states, one call each:
 `route_proxy_routes` (every served name with its target, certificate expiry and last answer), `route_proxy_route`