Pull request: 3185 detecting recursion

Merge in DNS/adguard-home from 3185-recursion to master

Closes #3185.

Squashed commit of the following:

commit 2fa44223f533c471f2b8c0e17d8550bf4ff73c7b
Merge: 7975957c 7a48e92e
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Thu May 27 19:04:44 2021 +0300

    Merge branch 'master' into 3185-recursion

commit 7975957cceb840f76eef0e2e434f4163a122ac34
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Thu May 27 17:36:22 2021 +0300

    dnsforward: imp docs

commit 1af7131a5b7c1fefed2d1eb8ee24ebfd3602dc77
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Thu May 27 17:15:00 2021 +0300

    dnsforward: imp code, tests, docs

commit f3f9145fb5e1174fab87ca6890da9df722cfebf0
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Thu May 27 15:45:44 2021 +0300

    dnsforward: add recursion detector

CHANGELOG.md

@@ -15,6 +15,7 @@ and this project adheres to
 
 ### Added
 
+- Detection and handling of recurrent requests ([#3185]).
 - The ability to completely disable reverse DNS resolving of IPs from
   locally-served networks ([#3184]).
 - New flag `--local-frontend` to serve dinamically changeable frontend files
@@ -38,6 +39,7 @@ released by then.
 - Go 1.15 support.
 
 [#3184]: https://github.com/AdguardTeam/AdGuardHome/issues/3184
+[#3185]: https://github.com/AdguardTeam/AdGuardHome/issues/3185