[dnsdist] [DoH] Cache control

[Stephane Bortzmeyer]

By default, dnsdist 1.4.0 working as a DoH resolver does not send
cache control headers (Cache-control:, Expires:, Last-Modified:…)

< HTTP/2 200 
< server: h2o/dnsdist
< content-type: application/dns-message
< content-length: 88

I do not find how to enable them. The RFC 8484 (section 5.1)
recommends Cache-control: based on the TTL.

cache-control = max-age=3709