Fix serve-stale hang at shutdown

Fix serve-stale hang at shutdown

The 'refresh_rrset' variable is used to determine if we can detach from
the client. This can cause a hang on shutdown. To fix this, move setting
of the 'nodetach' variable up to where 'refresh_rrset' is set (in
query_lookup(), and thus not in ns_query_done()), and set it to false
when actually refreshing the RRset, so that when this lookup is
completed, the client will be detached.