also see other ICMP implementations
http://godoc.org/golang.org/x/net/icmp
http://godoc.org/golang.org/x/net/ipv4#ICMPType
http://godoc.org/golang.org/x/net/ipv6#ICMPType

see
libbeat/CONTRIBUTING.md
libbeat/common/endpoint.go
libbeat/common/tuples.go
SUPPORT_PROTOCOL.md
docs/new_protocol.asciidoc
etc/fields.yml

requirements
- transaction: single message or message pair
- correlation key: ip: src, dst; icmp: id, seq
