update to conduit 4.0.0

[hannesm]

This is an attempt (where the resulting unikernel works) to port it to conduit 4.0.0.

There are some drawbacks: due to API changes, I'm completely lost how a with_tcp and with_tls could be reconstructed (per-session wrapper that emit a log message for each connection, including the IP address) -- maybe @samoht or @dinosaure know of a better path here?

The resulting unikernel supports both IPv4 and IPv6 \o/