Address TSAN error between dns_rbt_findnode() and subtractrdataset().

Having dns_rbt_findnode() in previous_closest_nsec() check of node->data is a optimisation that triggers a TSAN error with subtractrdataset(). find_closest_nsec() still needs to check if the NSEC record are active or not and look for a earlier NSEC records if it isn't. Set DNS_RBTFIND_EMPTYDATA so node->data isn't referenced without the node lock being held.

WARNING: ThreadSanitizer: data race
Read of size 8 at 0x000000000001 by thread T1 (mutexes: read M1, read M2):
#0 dns_rbt_findnode lib/dns/rbt.c:1708
#1 previous_closest_nsec lib/dns/rbtdb.c:3760
#2 find_closest_nsec lib/dns/rbtdb.c:3942
#3 zone_find lib/dns/rbtdb.c:4091
#4 dns_db_findext lib/dns/db.c:536
#5 query_lookup lib/ns/query.c:5582
#6 ns__query_start lib/ns/query.c:5505
#7 query_setup lib/ns/query.c:5229
#8 ns_query_start lib/ns/query.c:11380
#9 ns__client_request lib/ns/client.c:2166
#10 processbuffer netmgr/tcpdns.c:230
#11 dnslisten_readcb netmgr/tcpdns.c:309
#12 read_cb netmgr/tcp.c:832
#13 <null> <null>
#14 <null> <null>

Previous write of size 8 at 0x000000000001 by thread T2 (mutexes: write M3):
#0 subtractrdataset lib/dns/rbtdb.c:7133
#1 dns_db_subtractrdataset lib/dns/db.c:742
#2 diff_apply lib/dns/diff.c:368
#3 dns_diff_apply lib/dns/diff.c:459
#4 do_one_tuple lib/dns/update.c:247
#5 update_one_rr lib/dns/update.c:275
#6 delete_if_action lib/dns/update.c:689
#7 foreach_rr lib/dns/update.c:471
#8 delete_if lib/dns/update.c:716
#9 dns_update_signaturesinc lib/dns/update.c:1948
#10 receive_secure_serial lib/dns/zone.c:15637
#11 dispatch lib/isc/task.c:1152
#12 run lib/isc/task.c:1344
#13 <null> <null>

Location is heap block of size 130 at 0x000000000028 allocated by thread T3:
#0 malloc <null>
#1 default_memalloc lib/isc/mem.c:713
#2 mem_get lib/isc/mem.c:622
#3 mem_allocateunlocked lib/isc/mem.c:1268
#4 isc___mem_allocate lib/isc/mem.c:1288
#5 isc__mem_allocate lib/isc/mem.c:2453
#6 isc___mem_get lib/isc/mem.c:1037
#7 isc__mem_get lib/isc/mem.c:2432
#8 create_node lib/dns/rbt.c:2239
#9 dns_rbt_addnode lib/dns/rbt.c:1202
#10 dns_rbtdb_create lib/dns/rbtdb.c:8668
#11 dns_db_create lib/dns/db.c:118
#12 receive_secure_db lib/dns/zone.c:16154
#13 dispatch lib/isc/task.c:1152
#14 run lib/isc/task.c:1344
#15 <null> <null>

Mutex M1 (0x000000000040) created at:
#0 pthread_rwlock_init <null>
#1 isc_rwlock_init lib/isc/rwlock.c:39
#2 dns_rbtdb_create lib/dns/rbtdb.c:8527
#3 dns_db_create lib/dns/db.c:118
#4 receive_secure_db lib/dns/zone.c:16154
#5 dispatch lib/isc/task.c:1152
#6 run lib/isc/task.c:1344
#7 <null> <null>

Mutex M2 (0x000000000044) created at:
#0 pthread_rwlock_init <null>
#1 isc_rwlock_init lib/isc/rwlock.c:39
#2 dns_rbtdb_create lib/dns/rbtdb.c:8600
#3 dns_db_create lib/dns/db.c:118
#4 receive_secure_db lib/dns/zone.c:16154
#5 dispatch lib/isc/task.c:1152
#6 run lib/isc/task.c:1344
#7 <null> <null>

Mutex M3 (0x000000000046) created at:
#0 pthread_rwlock_init <null>
#1 isc_rwlock_init lib/isc/rwlock.c:39
#2 dns_rbtdb_create lib/dns/rbtdb.c:8600
#3 dns_db_create lib/dns/db.c:118
#4 receive_secure_db lib/dns/zone.c:16154
#5 dispatch lib/isc/task.c:1152
#6 run lib/isc/task.c:1344
#7 <null> <null>

Thread T1 (running) created by main thread at:
#0 pthread_create <null>
#1 isc_thread_create pthreads/thread.c:73
#2 isc_nm_start netmgr/netmgr.c:232
#3 create_managers bin/named/main.c:909
#4 setup bin/named/main.c:1223
#5 main bin/named/main.c:1523

Thread T2 (running) created by main thread at:
#0 pthread_create <null>
#1 isc_thread_create pthreads/thread.c:73
#2 isc_taskmgr_create lib/isc/task.c:1434
#3 create_managers bin/named/main.c:915
#4 setup bin/named/main.c:1223
#5 main bin/named/main.c:1523

Thread T3 (running) created by main thread at:
#0 pthread_create <null>
#1 isc_thread_create pthreads/thread.c:73
#2 isc_taskmgr_create lib/isc/task.c:1434
#3 create_managers bin/named/main.c:915
#4 setup bin/named/main.c:1223
#5 main bin/named/main.c:1523

SUMMARY: ThreadSanitizer: data race lib/dns/rbt.c:1708 in dns_rbt_findnode

(cherry picked from commit 244f84a8)

Closes #2211 (closed)