fix(nslord): create keys when quickly recreating a deleted zone

Thanks @Habbie for the suggestion.

related: #106

nslord/conf/pdns.conf.var

@@ -29,3 +29,4 @@ gmysql-dnssec=yes
 cache-ttl=${DESECSTACK_NSLORD_CACHE_TTL}
 negquery-cache-ttl=${DESECSTACK_NSLORD_CACHE_TTL}
 query-cache-ttl=${DESECSTACK_NSLORD_CACHE_TTL}
+dnssec-key-cache-ttl=0