double-attach when prefetching

@jtatuya pointed out in a comment on #2122 (closed) that there may be an issue with a handle being attached when non-NULL:

It's just based on code inspection so I'd apologize in advance if I'm mistaken. As a result of this change, fetchhandle structure member was introduced in ns_client and it can be passed to isc_nmhandle_attach from these three functions:

• ns_query_recurse
• query_rpzfetch
• query_prefetch

Is this safe? If I understand the implementation correctly, ns_query_recurse can be called while prefetching is taking place, so I suspect it's possible that isc_nmhandle_attach can be called with a non-NULL ns_client:fetchhandle. If that can actually happen that would trigger an assertion failure in isc_nmhandle_attach.