cache: default to DNSSEC (#4085)

* cache: default to DNSSEC

This change does away with the DNS/DNSSEC distinction the cache
currently makes. Cache will always make coredns perform a DNSSEC query
and store that result. If a client just needs plain DNS, the DNSSEC
records are stripped from the response.

It should also be more memory efficient, because we store a reply once
and not one DNS and another for DNSSEC.

Fixes: #3836

Signed-off-by: Miek Gieben <miek@miek.nl>

* Change OPT RR when one is present in the msg.

Signed-off-by: Miek Gieben <miek@miek.nl>

* Fix comment for isDNSSEC

Signed-off-by: Miek Gieben <miek@miek.nl>

* Update plugin/cache/handler.go

Co-authored-by: Chris O'Haver <cohaver@infoblox.com>

* Update plugin/cache/item.go

Co-authored-by: Chris O'Haver <cohaver@infoblox.com>

* Code review; fix comment for isDNSSEC

Signed-off-by: Miek Gieben <miek@miek.nl>

* Update doc and set AD to false

Set Authenticated Data to false when DNSSEC was not wanted. Also update
the readme with the new behavior.

Signed-off-by: Miek Gieben <miek@miek.nl>

* Update plugin/cache/handler.go

Co-authored-by: Chris O'Haver <cohaver@infoblox.com>

Co-authored-by: Chris O'Haver <cohaver@infoblox.com>

request/request.go

@@ -144,7 +144,7 @@ func (r *Request) Family() int {
 	return 2
 }
 
-// Do returns if the request has the DO (DNSSEC OK) bit set.
+// Do returns true if the request has the DO (DNSSEC OK) bit set.
 func (r *Request) Do() bool {
 	if r.size != 0 {
 		return r.do