net: add seed peers on every start

[octobocto]

Net::new wrote the seed addresses only in the None arm of DatabaseUnique::open, so a datadir that already holds known_peers never learns a seed that a later release names. On alphanet the stored seeds stopped answering, and those nodes found no peers at all. ensure_seed_peers now writes each seed the network names on every start, and it drops no stale one.