fix: better creation of RNTI
The previous version was calling taus(), which may return any value in [0..65535]. The new version restricts it to [1024..59999] (arbitrary values, big enough, and in the valid range). It also checks that the new RNTI is not already present in the MAC. I think we should also check that it's not used by a RA process. The "loop" thing may not be the best solution either. To be refined.