fix the cache findzonecut implementation

fix the cache findzonecut implementation

the search for the deepest known zone cut in the cache could
improperly reject a node containing stale data, even if the
NS rdataset wasn't the data that was stale.

this change also improves the efficiency of the search by
stopping it when both NS and RRSIG(NS) have been found.

(cherry picked from commit 1f095b902c834fd7bf0805605548ef8ca14c5af3)