dig: add reference counter to the dig_lookup_t object

dig: add reference counter to the dig_lookup_t object

Sometimes, the dig_lookup_t could be destroyed before the final
send_done() callback was be called, leading to dereferencing an
already freed dig_lookup_t object.  By making the dig_lookup_t
reference counted, we are ensuring that it won't be freed until
the last reference (from dig_query_t .lookup) is released.