Pull request: all: simplify dnssec logic

Closes #3904.

Squashed commit of the following:

commit 5948f0d3519299a1253e388f4bc83e2e55847f68
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Mon Dec 13 17:53:40 2021 +0300

    querylog: imp

commit 852cc7dbdb495a17ff51b99ab12901b846f8be09
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Mon Dec 13 17:44:41 2021 +0300

    querylog: fix entry write

commit 9d58046899f35162596bfc94fe88fa944309b2fd
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Mon Dec 13 16:45:56 2021 +0300

    all: simplify dnssec logic

openapi/openapi.yaml

@@ -1878,6 +1878,8 @@
             Upstream URL starting with tcp://, tls://, https://, or with an IP
             address.
         'answer_dnssec':
+          'description': >
+            If true, the response had the Authenticated Data (AD) flag set.
           'type': 'boolean'
         'client':
           'description': >