Refreshing a stale RRset is similar to prefetching an RRset, so
reuse the existing code. When refreshing an RRset we need to clear
all db options related to serve-stale so that stale RRsets in cache
are ignored during the refresh.
We no longer need to set the "nodetach" flag, because the refresh
fetch is now a "fetch and forget". So we can detach from the client
in the query_send().
This code will break some serve-stale test cases, this will be fixed
in the successor commit.
TODO: add explanation why the serve-stale test cases fail.
64d51285d5