resolved: limit the number of signature validations in a transaction

resolved: limit the number of signature validations in a transaction

It has been demonstrated that tolerating an unbounded number of dnssec
signature validations is a bad idea. It is easy for a maliciously
crafted DNS reply to contain as many keytag collisions as desired,
causing us to iterate every dnskey and signature combination in vain.

The solution is to impose a maximum number of validations we will
tolerate. While collisions are not hard to craft, I still expect they
are unlikely in the wild so it should be safe to pick fairly small
values.

Here two limits are imposed: one on the maximum number of invalid
signatures encountered per rrset, and another on the total number of
validations performed per transaction.