traffic: generate sparser descriptions of conversations

traffic: generate sparser descriptions of conversations

Rather than building all the packets at this point, we stick to the
barest details of the packets (which is all the model gives us
anyway).

The advantage is that will take a lot less memory, which matters
because this process forks into many clients that were sharing and
mutate the conversation list.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>