Use a reference counter for zt

Use a reference counter for zt

WARNING: ThreadSanitizer: data race
  Write of size 8 at 0x000000000001 by thread T1 (mutexes: write M1):
    #0 memset <null>
    #1 mem_put lib/isc/mem.c:819
    #2 isc___mem_free lib/isc/mem.c:1662
    #3 isc__mem_free lib/isc/mem.c:3078
    #4 isc___mem_putanddetach lib/isc/mem.c:1221
    #5 isc__mem_putanddetach lib/isc/mem.c:3033
    #6 zt_destroy lib/dns/zt.c:214
    #7 doneloading lib/dns/zt.c:591
    #8 zone_asyncload lib/dns/zone.c:2243
    #9 dispatch lib/isc/task.c:1157
    #10 run lib/isc/task.c:1331
    #11 <null> <null>

  Previous atomic read of size 8 at 0x000000000001 by thread T2:
    #0 __tsan_atomic64_load <null>
    #1 isc_rwlock_unlock lib/isc/rwlock.c:612
    #2 doneloading lib/dns/zt.c:585
    #3 zone_asyncload lib/dns/zone.c:2243
    #4 dispatch lib/isc/task.c:1157
    #5 run lib/isc/task.c:1331
    #6 <null> <null>

  Location is heap block of size 273 at 0x000000000015 allocated by thread T3:
    #0 malloc <null>
    #1 internal_memalloc lib/isc/mem.c:887
    #2 mem_get lib/isc/mem.c:792
    #3 mem_allocateunlocked lib/isc/mem.c:1545
    #4 isc___mem_allocate lib/isc/mem.c:1566
    #5 isc__mem_allocate lib/isc/mem.c:3048
    #6 isc___mem_get lib/isc/mem.c:1304
    #7 isc__mem_get lib/isc/mem.c:3012
    #8 dns_zt_create lib/dns/zt.c:85
    #9 dns_view_create lib/dns/view.c:126
    #10 create_view server.c:5312
    #11 load_configuration server.c:8101
    #12 loadconfig server.c:9428
    #13 ns_server_reconfigcommand server.c:9763
    #14 ns_control_docommand bin/named/control.c:243
    #15 control_recvmessage bin/named/controlconf.c:465
    #16 dispatch lib/isc/task.c:1157
    #17 run lib/isc/task.c:1331
    #18 <null> <null>