mitigate NXNSAttack protocol vulnerability for wildcards in victim zone

mitigate NXNSAttack protocol vulnerability for wildcards in victim zone

Attacker might generate fake NS records pointing to victim's DNS zone.
If the zone contains wildcard the attacker might force us into packet
exchange with a (lame) DNS server on that IP address.

We now limit number of consecuctive failures and kill whole request if
limit is exceeded.