* p2p/enr: add entries for for IPv4/IPv6 separation
This adds entry types for "ip6", "udp6", "tcp6" keys. The IP type stays
around because removing it would break a lot of code and force everyone
to care about the distinction.
* p2p/enode: track IPv4 and IPv6 address separately
LocalNode predicts the local node's UDP endpoint and updates the record.
This change makes it predict IPv4 and IPv6 endpoints separately since
they can now be in the record at the same time.
* p2p/enode: implement base64 text format
* all: switch to enode.Parse(...)
This allows passing base64-encoded node records to all the places that
previously accepted enode:// URLs. The URL format is still supported.
* cmd/bootnode, p2p: log node URL instead of ENR
...and return the base64 record in NodeInfo.
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