Expiry of discovered peers when they are no longer alive

[andrewdavidmackenzie]

When a peer has been discovered and appears in the remotes of the "discoveree", it stays there for a long time (I can't tell if "forever", or just a very long time), well after the peer has closed/crashed/ended.

The suggested method of using last_used to filter out peers discovered some time ago doesn't work:

Docs
https://docs.rs/iroh-net/latest/iroh_net/endpoint/struct.RemoteInfo.html

It seems last_used field (not called last_seen) is only updated when data transfer between peers takes place, not when that peer is discovered, or "re-discovered". It returns None in discovered peers in my tests, and that seems to match the docs description also.

So, I think we'd either need some other method of expiring peers (mDNS has a TTL setting when registering a service), or this field would need to be redefined, and updated each time the peer is discovered/confirmed-alive. But I'd argue that of the TTL expires and we try to confirm-alive and the peer is not alive, then it should be removed from the remotes - and updating the last_used would not be useful.