isc_nmhandle_detach() needs to complete in the same thread
as shutdown_walk_cb() to avoid a race.  Clear the caller's
pointer then pass control to the worker if necessary.

    WARNING: ThreadSanitizer: data race
    Write of size 8 at 0x000000000001 by thread T1:
    #0 isc_nmhandle_detach lib/isc/netmgr/netmgr.c:1258:15
    #1 control_command bin/named/controlconf.c:388:3
    #2 dispatch lib/isc/task.c:1152:7
    #3 run lib/isc/task.c:1344:2

    Previous read of size 8 at 0x000000000001 by thread T2:
    #0 isc_nm_pauseread lib/isc/netmgr/netmgr.c:1449:33
    #1 recv_data lib/isccc/ccmsg.c:109:2
    #2 isc__nm_tcp_shutdown lib/isc/netmgr/tcp.c:1157:4
    #3 shutdown_walk_cb lib/isc/netmgr/netmgr.c:1515:3
    #4 uv_walk <null>
    #5 process_queue lib/isc/netmgr/netmgr.c:659:4
    #6 process_normal_queue lib/isc/netmgr/netmgr.c:582:10
    #7 process_queues lib/isc/netmgr/netmgr.c:590:8
    #8 async_cb lib/isc/netmgr/netmgr.c:548:2
    #9 <null> <null>