Port last changes of discv4 from go-ethereum

[AskAlexSharov]

Next changes need to port:

• p2p/discover/v4wire: use optional RLP field for EIP-868 seq ethereum/go-ethereum#22842
• p2p/discover: avoid dropping unverified nodes when table is almost empty ethereum/go-ethereum#21396

[battlmonstr]

21396 is already applied here

[battlmonstr]

done on devel @ 13d6619