too early, while it could still be in use by queries.
This manifested itself as assertion failures during the
shutdown process for busy name servers (RT #133).
256. [func] isc_ratelimiter_t now has attach/detach semantics, and
isc_ratelimiter_shutdown guarantees that the rate
limiter is detached from its task.
255. [func] New function dns_zonemgr_attach().