rec: Defer the NOD lookup until after the response has been sent

rec: Defer the NOD lookup until after the response has been sent

If the NOD lookup is slow, for example because the destination
authoritative server is down, doing the NOD lookup before the response
has been sent increases the latency a lot.
This commit moves the actual NOD lookup after the response has been
sent, so we can still use the existing mthread (we might actually need
to do a proper DNS resolution to find the target authoritative server)
without keeping the client waiting.