DNS: refactor tx completion logic

DNS: refactor tx completion logic

Use simple bool values to track the transaction state in both directions.

A tx is only created in two cases:
1. full request parsed
2. response parsed (request missing)

This is true even for multi-packet TCP requests.

This leads to the following tx completion logic for the request side:
the presence of a tx implies the request is complete

On the response side, we consider the tx complete when we have seen
the response. If the DNS parser thinks the response was lost, we also
flag the response side as complete.