This change makes information in ENR compatible with the existing
discovery protocol:
- The default identity scheme is now called "v4" to make the name shorter.
- Node addresses are derived as the hash of the uncompressed public key.
- The "ip4" and "ip6" keys are merged into a single "ip" key which
can hold a IPv4 or IPv6 address.
- The "discv5" key is now called "udp".
- The new "tcp" key holds the TCP port.