Don't use rwlocks for reference counting

Don't use rwlocks for reference counting

    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 destroyring lib/dns/tsig.c:494
    #7 dns_tsigkeyring_dumpanddetach lib/dns/tsig.c:665
    #8 destroy lib/dns/view.c:392
    #9 dns_view_weakdetach lib/dns/view.c:704
    #10 zone_free lib/dns/zone.c:1152
    #11 zone_shutdown lib/dns/zone.c:13123
    #12 dispatch lib/isc/task.c:1157
    #13 run lib/isc/task.c:1331
    #14 <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 dns_tsigkeyring_dumpanddetach lib/dns/tsig.c:632
    #3 destroy lib/dns/view.c:392
    #4 dns_view_weakdetach lib/dns/view.c:704
    #5 zone_free lib/dns/zone.c:1149
    #6 zone_shutdown lib/dns/zone.c:13123
    #7 dispatch lib/isc/task.c:1157
    #8 run lib/isc/task.c:1331
    #9 <null> <null>