Add separate prefetch nmhandle to ns_client_t

Add separate prefetch nmhandle to ns_client_t

As the query_prefetch() or query_rpzfetch() could be called during
"regular" fetch, we need to introduce separate storage for attaching
the nmhandle during prefetching the records.  The query_prefetch()
and query_rpzfetch() are guarded for re-entrance by .query.prefetch
member of ns_client_t, so we can reuse the same .prefetchhandle for
both.