TSAN errors in v9_16 to be investigated.

Job #1192145 failed for fc3cab22:

I:System test result summary:
I:      2 FAIL
I:     94 PASS
I:      3 SKIPPED
I:The following system tests failed:
I:      nsupdate
I:      tcp
I:ThreadSanitizer reported issues for the following system tests:
I:      tcp

Races over access to sock->rcb.recv and sock->rcbarg. sock->rcbarg in this case.

sock->rcbarg is set to NULL before calling isc_nm_stoplistening. Additionally we need to wait for isc_nm_stoplistening to be processed when it is sent via the event queue.

WARNING: ThreadSanitizer: data race 
  Read of size 8 at 0x000000000001 by thread T1:
    #0 processbuffer lib/isc/netmgr/tcpdns.c:185:15
    #1 resume_processing lib/isc/netmgr/tcpdns.c:421:12
    #2 isc_nmhandle_unref lib/isc/netmgr/netmgr.c:1173:4
    #3 isc__nm_uvreq_put lib/isc/netmgr/netmgr.c:1288:3
    #4 tcpdnssend_cb lib/isc/netmgr/tcpdns.c:449:2
    #5 tcp_send_cb lib/isc/netmgr/tcp.c:892:2
    #6 <null> <null>

  Previous write of size 8 at 0x000000000001 by thread T2 (mutexes: write M1):
    #0 memset <null>
    #1 isc__nm_tcpdns_stoplistening lib/isc/netmgr/tcpdns.c:330:15
    #2 isc_nm_stoplistening lib/isc/netmgr/netmgr.c:1358:3
    #3 ns_interface_shutdown lib/ns/interfacemgr.c:564:3
    #4 purge_old_interfaces lib/ns/interfacemgr.c:658:4
    #5 ns_interfacemgr_shutdown lib/ns/interfacemgr.c:386:2
    #6 shutdown_server bin/named/./server.c:9851:2
    #7 dispatch lib/isc/task.c:1152:7
    #8 run lib/isc/task.c:1344:2