ddm: consider proper interface scope handling for link-local peers

While working on some updates to FSM state tracking for DDM, I observed that ddmadm expire-peer takes an IP as an argument but no interface (name, addrobj, ifindex, or any other representation) even though we peer over link-local addresses.

That seemed odd to me so I dug a little bit further.

The expire-peers operation triggers an AdminEvent::Expire(addr) event that's sent to every interface FSM, who then compares that addr against their peer addr. The event wraps just an Ipv6Addr so it doesn't have any scope_id included, nor does it filter the FSM event just to one interface FSM that corresponds to the (IP, scope) tuple.

Additionally, routes are stored in the DB with just a peer address and no next-hop interface, which presumably means we are relying on NDP for installation to work properly.

While this has all worked well in practice, it would probably be a good idea to ensure proper handling of the same link-local IPs learned via multiple interfaces to be more robust in the face of an unforeseen issue (e.g. improper MAC assignment during sled manufacturing).

Start by tracing the ddmadm expire-peer operation to AdminEvent::Expire(addr) and the interface FSMs that consume it. Review how peer routes are stored in the DB and how link-local addresses are represented across interfaces. Done means the expire operation and route handling distinguish the same link-local address on different interfaces, with coverage for the relevant behavior.