Add missing lock calls in isc__nm_tcpdns_stoplistening()

isc__nmsocket_clearcb() can update a socket being processed
on a different thread and without the lock calls tsan errors
can be produced between isc__nm_tcpdns_stoplistening() and
processbuffer().
74 jobs for !4215 with marka-missing-lock-in-isc__nm_tcpdns_stoplistening in 19 minutes and 31 seconds (queued for 7 seconds)
merge request