_query_detach function was incorrectly unliking the query object from
the lookup->q query list, this made it impossible to follow a query
lookup failure with the next one in the list (possibly using a separate
resolver), as the link to the next query in the list was dissolved.
Fix by unliking the node only when the query object is about to be
destroyed, i.e. there is no more references to the object.
4d6408b823