This change implements EIP-868. The UDPv4 transport announces support
for the extension in ping/pong and handles enrRequest messages.
There are two uses of the extension: If a remote node announces support
for EIP-868 in their pong, node revalidation pulls the node's record.
The Resolve method requests the record unconditionally.
This change restructures the internals of p2p/discover to make room for
the discv5 code which will soon be added to this package.
- packet type names now have a "V4" suffix.
- ListenUDP returns *UDPv4 instead of *Table. This technically breaks
the API but the only caller in go-ethereum is package p2p, which uses
a compatible interface and doesn't need changes.
- The internal transport interface is changed to make Table reusable for v5.
- The 'lookup' code moves from table to transport. This required
updating the lookup unit test to use udpTest instead of a custom transport.
