add an option to default to TCP dns queries

[Tieske]

dns queries are by default done using UDP, only when truncation happens it will fall back to TCP. Some tools will only return a limited number of entries for UDP queries (Consul for example).

By using TCP we could get the full list and do proper loadbalancing over all of them.

Original request: https://discuss.konghq.com/t/seeking-additional-details-around-dns-resolution/1997/4?u=tieske

[Tieske]

This requires TCP support from the resty dns lib. See openresty/lua-resty-dns#40

[creslinux]

Hi, In and around this thread

consul has an option to return the truncate flag when the return payload will be greater than 512bytes.
This should be standard compliant behaviour, but consul need the setting flagged to be so.

hashicorp/consul#376

[creslinux]

Made a PR for documentation clarification. #1835
Kong/docs.konghq.com#1835