Hold a reference to the NTA table for the lifetime of each NTA

Hold a reference to the NTA table for the lifetime of each NTA

Each dns__nta_t now references its parent ntatable in nta_create() and
releases it in dns__nta_destroy().  This avoids a use-after-free in
fetch_done() and other callbacks that dereference nta->ntatable: the
ntatable could otherwise be released by view destruction while an
in-flight resolver fetch still holds a reference to the NTA.