A DHT implementation for Codex

This DHT implementation is aiming to provide a DHT for Codex with the following properties

flexible secure transport usage with
- fast UDP based operation
- eventual fallback to TCP-based operation (maybe though libp2p)
- eventually support operation on top of libp2p
flexible message encoding that plays well with the above transports
provide node lookup, content storage/lookup, and provider storage/lookup operations
- roughly follow the libp2p-dht specifications from https://github.com/libp2p/specs/tree/master/kad-dht
- eventually provide compatibility mode with the above specs

Current implementation is based on nim-eth's Discovery v5 implementation.