Validate route origins against the RPKI and refuse invalid routes

With rpki { caches { ... } } in the configuration the server keeps the
validated ROA payloads of a validator and validates every route from a
peer outside its own AS (RFC 6811). An invalid route is refused, unless
its peer has accept_rpki_invalid. Valid and not-found routes are taken;
validity does not otherwise enter route selection.

Validity is not settled when a route arrives: the data changes every few
minutes. The routes follow, without the peer sending anything again.

- A route refused as invalid is held by its session, out of the table.
  When the data changes the session looks at what it holds, and what is
  no longer invalid goes into the table. A held route is the peer's to
  withdraw or replace like any other.
- A route in the table that becomes invalid is found by a task that
  looks through the table when the data changes, at the prefixes the
  changed VRPs cover, and is taken out and held by its session.
- If the data expires because no cache confirms it, nothing is invalid
  any more and everything held is taken.
- A session holds at most 200,000 routes; past that it asks the peer for
  a route refresh when the data changes.

So that the table can be looked through without parsing attributes, an
attribute set's key now has the route's origin AS. Memory per route is
unchanged at 52 bytes on the real table.

Shown: /api/rpki and a section of the status page with the cache, its
state and the age of the data; each route's validity; each peer's count
of held routes. Refusals are recorded under "RPKI invalid". Metrics:
bgp_rpki_vrps and bgp_rpki_updates_total.

Also: Negotiated says whether the peer supports route refresh, and
docs/RFCs.md is brought up to date.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

testdata/server.textpb

@@ -53,6 +53,10 @@ originate {
     large_communities: "65000:1:2"
 }
 
+# Route origin validation: refuse routes whose origin AS is not the one the
+# holder of the addresses authorised. Point it at a validator's RTR server.
+# rpki { caches { address: "127.0.0.1" port: 3323 } }
+
 # Read-only status page, JSON API and metrics. It has no authentication:
 # keep it on loopback or behind a reverse proxy.
 http_endpoint { address: "127.0.0.1" port: 8080 }