rec: Keep a cached, valid entry over a fresher Bogus one

rec: Keep a cached, valid entry over a fresher Bogus one

It turns out to be quite difficult to make us accept a record that
we already have in cache, thanks to sanitization, but let's make
sure that we will not replace a valid entry with a Bogus one if that
happens.
It might happen for SOA records, and for DS records when the TTL of
the corresponding NS records is shorter than the TTL of the DS.