Fix concurrency issues happening when a new client connects, sometimes
leading to a total softmodem deadlock.
Key changes:
- Fix sample sending on new client connection: send the first
sample only to the newly connected client.
- Remove the --rfsimulator.hanging-workaround option
- Replace global nb_ue and Sockmutex with nb_cnx and Sockmutex
in rfsimulator_state_t for better concurrency and clarity.