This prevents issues with concurrent access to the ringbuf.
MICROPY_BEGIN_ATOMIC_SECTION is only atomic to the same core. We could
address this with a mutex, but it's also not safe to call mp_sched_schedule
across cores.
This avoids a confusing ENOMEM raised from gap_advertise if there is
currently an active connection. This refers to the static connection
buffer pre-allocated by Nimble (nothing to do with MicroPython heap
memory).
