[Tracking] Implement `discv4` to connect to p2p peers

[mpaulucci]

We need this protocol to be able to connect to peers.

See: https://github.com/ethereum/devp2p/blob/master/discv4.md

Here's a reference implementation: https://github.com/akula-bft/akula/tree/master/src/sentry/devp2p/disc/v4

Related issues:

• Implement Ping message encoding #74
• Implement Ping message decoding #79
• Implement Pong message encoding #80
• Implement Pong message decoding #81
• Implement FindNode message encoding #82
• Implement FindNode message decoding #83
• Implement Neighbors message encoding #84
• Implement Neighbors message decoding #85
• Connect to bootnodes and request peer data #86
• Store peer data #87
• Implement ENRRequest message encoding #88
• Implement ENRRequest message decoding #89
• Implement ENRResponse message encoding #90
• Implement ENRResponse message decoding #91