AddrFetch connections (old name: oneshots) are intended to be short-lived connections on which we ask a peer for addresses via getaddr and disconnect after receiving them. This is done by disconnecting after receiving the first addr. However, it is no longer working as intended, because nowadays, the first addr a typical bitcoin core node sends is its self-announcement. So we'll disconnect before the peer gets a chance to answer our getaddr. I checked that this affects both -seednode peers specified manually, and DNS seeds when AddrFetch is used as a fallback if DNS doesn't work for us. The current behavior of getting peers via AddrFetch when starting with an empty addrman would be to connect to the peer, receive its self-announcement and add it to addrman, disconnect, reconnect to the same peer again as a full outbound (no other addresses in addrman) and then receive more addr. This is silly and not in line with AddrFetch peer being intended to be short-lived peers. Fix this by only disconnecting after receiving an addr message of size > 1. [Edit] As per review discussion, this PR now also adds a timeout after which we disconnect if we haven't received any suitable addr, and a functional test.
Backport of core#22096.
Ref T1696.