Route reflection, with ourselves as next hop

A route learned from one router of our AS went to no other (RFC 4271
§9.2.1), so every router needed a session with every other. A router at
the end of a single link, with one session to its neighbour inside the AS,
heard only of what that neighbour learned itself, and nothing of what the
neighbour had from the routers behind it.

A peer in our own AS can now be made a route reflector client
(`route_reflector_client`, RFC 4456). What a client sends goes to all our
routers; what the others send goes to the clients. A reflected route gets
ORIGINATOR_ID (where it started, if it does not say) and our cluster ID at
the front of CLUSTER_LIST; it is not sent back to the router it started
at, nor round to a cluster it has been through, and one that comes back
anyway is refused as "route reflected back to us". The cluster ID is the
router ID unless `cluster_id` says otherwise. The two attributes are now
decoded as such rather than kept as bytes, and are never sent to other
networks.

Unlike a standard reflector, this one is the next hop of what it reflects,
as it is of everything it announces: there is no resolving a next hop
that is not a neighbour, so a client reaches what lies behind another
router through the reflector.

Only peers in our own AS can be clients; the configuration refuses the
setting for any other. Nothing changes in what other networks are told:
Only to Customer marks go along with reflected routes, and every
receiving session keeps its own max_prefixes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/bench/benches/full_table.rs

@@ -164,6 +164,7 @@ fn into_table(wire: &[Vec<BytesMut>], routes: usize, threads: usize) {
                     address: Ipv4Addr::new(192, 0, 2, peer as u8 + 1).into(),
                     interface: 0,
                     ibgp: false,
+                    reflector_client: false,
                 })
                 .expect("room for the peer")
         })