Fix the forward zones in the recursor

Fix the forward zones in the recursor

In the pre-DNSName era, when dns-native names were passed as strings, we
overloaded the NS-name for a forward or auth zone. e.g. an empty string
meant 'this is an auth zone' and '+203.0.113.1' meant 'forward to 203.0.113.1
with the RD bit set'. With DNSNames, this is impossible (yay!).

In this commit, the set of strings (and later DNSNames), is replaced by
a map where a DNSName is the key and the value is a pair of a
ComboAddress and a boolean.

A non-empty DNSName: This is a normal NS, recurse as usual (the pair is
ignored).

An empty DNSName and empty ComboAddress: We are auth for this zone,
check the auth store for an answer.

An empty DNSName and non-empty ComboAddress: The query must be forwarded
to the ComboAddress specified and the boolean in the pair tells us the
value of the RD bit in the query we need to send.