nostr: back off from a relay that is down, and replace one that is (#64)

relay.nostr.band went down and stayed down -- it resolves and refuses TCP
on 443, from several networks. The seed accumulated 1092 identical error
lines about it.

The log noise was the small half. The discovery loop walks every relay in
turn with a ten-second connect timeout, so a dead relay added ten seconds
to every pass, forever, and that loop is what finds peers -- a relay that
answers nobody was slowing down the ones that answer everybody.

Back off per relay: 30s after the first failure, doubling to a fifteen
minute ceiling, cleared the moment it answers again. Only when an attempt
happens changes; nothing about whether discovery is correct.

Also swapped relay.nostr.band for nostr.bitcoiner.social, on 206.245.132.32
-- neither Cloudflare nor Hetzner, which is the diversity rule the list
already follows.

Verified with a deliberately bogus relay in the list: backoff stepped
30s -> 60s -> 120s and the dead entry produced nine lines in four minutes
instead of one per pass, while the node still connected to five relays,
discovered 54 fresh peers and followed the chain.

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