Add Conntrack support (NETLINK_NETFILTER protocol)

[2opremio]

It would be great if this library was extended to support conntrack

Ref: weaveworks/scope#1991

[2opremio]

Relevant: https://github.com/typetypetype/conntrack/ (which isn't complete, it lacks support for full flows)

[2opremio]

I don't know why this was automatically closed, reopening.

[ti-mo]

Would also love to see Conntrack implemented in this library. @2opremio are you still planning on doing this in Scope?

[2opremio]

Not for now. I worked around it by parsing the output of the conntrack command (the plain one, since parsing XML was much more expensive)

[fcrisciani]

@2opremio Check the latest master, I put some foundation for the netfilter conntrack support.

[2opremio]

Thanks @fcrisciani ! After #213 , getting events should be fairly easy! (I cannot commit to do it for now though).