Implement in-depth topology simulation via network model

[martinruefenacht]

The aim of this is to capture the congestion effects better, instead of mathematical models.

• [ ] Implement network abstraction
• [x] Implement network models

The network models should use the global priority queue to progress the simulation. They should not use a nested priority queue.

[martinruefenacht]

• [ ] network congestion
• [ ] NIC congestion (recv side, send side is already modelled)

[martinruefenacht]

recv side is done with a message task at the other side switch congestion, how?