Support not returning until the additional section is fully populated.

For SRV to be as useful as CNAME the additional section needs to be populated even when there was a cache miss for target servers A and AAAA records. Add code fetch these records before returning the answer to the SRV request. Additionally CNAME records need to be chased in the additional section. Additionally there needs to be a way to indicate that the additional section is complete.

Add a flag to enable this experimental behaviour. In future this should be triggered by a EDNS option indicating that the client wants the additional section to be populated.