validator: bottom->up chase DS if RRSIG(s) are missing

This is about situations when validator *thinks* it's in a signed zone
but an unsigned answer comes in. The assumption was that RRSIGs didn't
make it through some middle-boxes and it retried with explicit QTYPE=RRSIG.

There were two issues with that.
1. It seems that in most cases the cause of the situation is that
   we skipped over a zone cut that transitioned to insecure state,
   so the signatures correctly don't exist.
2. An explicit RRSIG query appears to be more trouble than worth;
   it seems reasonable for servers not to answer it (fully);
   see RFC 8482 sect. 7.

The new approach simply tries to find a proof that the name is insecure,
by spawning a QTYPE=DS sub-query on that name.  That fixes some
real-life cases; usually this happens in iteration mode where one IP
address serves zones on both sides of a cut that transitions to insecure.
For details see new comments in that rrsig_not_found() function.

The change resulted in the iterator fallback not making sense anymore
so it was removed.