Address lock-order-inversion

WARNING: ThreadSanitizer: lock-order-inversion (potential deadlock) Cycle in lock order graph: M1 (0x000000000001) => M2 (0x000000000002) => M1

Mutex M2 acquired here while holding mutex M1 in thread T1:
#0 pthread_rwlock_wrlock <null>
#1 isc_rwlock_lock lib/isc/rwlock.c:52:4
#2 zone_postload lib/dns/zone.c:5101:2
#3 receive_secure_db lib/dns/zone.c:16206:11
#4 dispatch lib/isc/task.c:1152:7
#5 run lib/isc/task.c:1344:2

Mutex M1 previously acquired by the same thread here:
#0 pthread_mutex_lock <null>
#1 receive_secure_db lib/dns/zone.c:16204:2
#2 dispatch lib/isc/task.c:1152:7
#3 run lib/isc/task.c:1344:2

Mutex M1 acquired here while holding mutex M2 in thread T1:
#0 pthread_mutex_lock <null>
#1 get_raw_serial lib/dns/zone.c:2518:2
#2 zone_gotwritehandle lib/dns/zone.c:2559:4
#3 dispatch lib/isc/task.c:1152:7
#4 run lib/isc/task.c:1344:2

Mutex M2 previously acquired by the same thread here:
#0 pthread_rwlock_rdlock <null>
#1 isc_rwlock_lock lib/isc/rwlock.c:48:3
#2 zone_gotwritehandle lib/dns/zone.c:2552:2
#3 dispatch lib/isc/task.c:1152:7
#4 run lib/isc/task.c:1344:2

Thread T1 (running) created by main thread at:
#0 pthread_create <null>
#1 isc_thread_create lib/isc/pthreads/thread.c:73:8
#2 isc_taskmgr_create lib/isc/task.c:1434:3
#3 create_managers bin/named/main.c:915:11
#4 setup bin/named/main.c:1223:11
#5 main bin/named/main.c:1523:2

SUMMARY: ThreadSanitizer: lock-order-inversion (potential deadlock) in pthread_rwlock_wrlock

(cherry picked from commit 10908766)

Closes #2157 (closed)