This change implements the Discovery v5.1 wire protocol and
also adds an interactive test suite for this protocol.
# Conflicts:
# cmd/devp2p/discv5cmd.go
# p2p/discover/v5_encoding.go
# p2p/discover/v5_encoding_test.go
# p2p/discover/v5_udp.go
# p2p/discover/v5_udp_test.go
# p2p/discover/v5wire/session.go