NAT session manager: Create new flows into flow manager

[qmonnet]

Once a new NAT session has been created, we need to ask the flow manager to create the resulting flows.

TO DO: Figure out how to create relevant flows in the case of stateless NAT, for which we don't record NAT session. I don't know how expressive are the flows we can generate (for stateless NAT, we don't have a 1:1 mapping for an existing connection, but instead we want to use a single algorithm to map automatically any packet).