Discv4: ENR support

[kdeme]

ENR support was added at a later stage to discv4.
Our current implementation does not have it.

ENR itself is already implemented (and used by discv5 implementation).

The issue is about adding support with is probably mostly about adding the ENR seqnum to ping and pong` packets: https://github.com/ethereum/devp2p/blob/master/discv4.md#ping-packet-0x01

And adding the new packet types ENRRequest and ENRResponse: https://github.com/ethereum/devp2p/blob/master/discv4.md#enrrequest-packet-0x05