Fix l2 sim

This fixes the L2 simulator: multiple UEs are correctly authenticated and reports the CQI (currently hard-coded but can be easily extended to random or pre-defined values). It has also been discovered that multiple threads cause segfaults (due to data races?).

With these patches, it is possible to have at least four UEs connected and iperf traffic in UL and DL.

I will make some more tests with 16+ UEs.