Lock access to flags in dns__zone_loadpending

Lock access to flags in dns__zone_loadpending

==================
WARNING: ThreadSanitizer: data race (pid=1938)
  Read of size 4 at 0x7b7800000dd4 by main thread:
    #0 dns__zone_loadpending /builds/isc-projects/bind9/lib/dns/zone.c:2308:10 (libdns.so.1110+0x1da342)
    #1 asyncload_zone /builds/isc-projects/bind9/lib/dns/tests/zt_test.c:204:9 (zt_test+0x4b8303)
    #2 <null> <null> (libcmocka.so.0+0x50d8)
    #3 __libc_start_main /build/glibc-vjB4T1/glibc-2.28/csu/../csu/libc-start.c:308:16 (libc.so.6+0x2409a)

  Previous write of size 4 at 0x7b7800000dd4 by thread T16 (mutexes: write M2181):
    #0 zone_asyncload /builds/isc-projects/bind9/lib/dns/zone.c:2237:3 (libdns.so.1110+0x1da08d)
    #1 dispatch /builds/isc-projects/bind9/lib/isc/task.c:1157:7 (libisc.so.1107+0x50845)
    #2 run /builds/isc-projects/bind9/lib/isc/task.c:1331:2 (libisc.so.1107+0x4d799)