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.