Lock access to acache stats when not using atomics

Lock access to acache stats when not using atomics

    WARNING: ThreadSanitizer: data race
    Write of size 4 at 0x000000000001 by thread T1 (mutexes: write M1):
    #0 dns_acache_getentry lib/dns/acache.c:1549:2
    #1 rdataset_getadditional lib/dns/rbtdb.c:9912:11
    #2 dns_rdataset_getadditional lib/dns/rdataset.c:711:11
    #3 query_addadditional2 bin/named/query.c:1991:11
    #4 additionaldata_ns lib/dns/./rdata/generic/ns_2.c:198:10
    #5 dns_rdata_additionaldata lib/dns/rdata.c:1246:2
    #6 dns_rdataset_additionaldata lib/dns/rdataset.c:629:12
    #7 query_addrdataset bin/named/query.c:2435:8
    #8 query_addrrset bin/named/query.c:2826:2
    #9 query_addbestns bin/named/query.c:3525:2
    #10 query_find bin/named/query.c:9204:4
    #11 query_resume bin/named/query.c:4188:12
    #12 dispatch lib/isc/task.c:1157:7
    #13 run lib/isc/task.c:1331:2

    Previous write of size 4 at 0x000000000001 by thread T2 (mutexes: write M2):
    #0 dns_acache_countquerymiss lib/dns/acache.c:1201:2
    #1 rdataset_getadditional lib/dns/rbtdb.c:9896:4
    #2 dns_rdataset_getadditional lib/dns/rdataset.c:711:11
    #3 query_addadditional2 bin/named/query.c:1991:11
    #4 additionaldata_ns lib/dns/./rdata/generic/ns_2.c:198:10
    #5 dns_rdata_additionaldata lib/dns/rdata.c:1246:2
    #6 dns_rdataset_additionaldata lib/dns/rdataset.c:629:12
    #7 query_addrdataset bin/named/query.c:2435:8
    #8 query_addrrset bin/named/query.c:2826:2
    #9 query_find bin/named/query.c:9176:4
    #10 query_resume bin/named/query.c:4188:12
    #11 dispatch lib/isc/task.c:1157:7