Pull request: querylog: more opt

Updates 1273.

Squashed commit of the following:

commit 167c0b5acaab8a2676de2cea556c861dc0efbc72
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Mon Apr 12 18:12:43 2021 +0300

    querylog: imp naming

commit 5010ad113e46335011a721cbcc9fc9b1fc623722
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Mon Apr 12 17:53:41 2021 +0300

    querylog: more opt

HACKING.md

@@ -243,6 +243,33 @@ on GitHub and most other Markdown renderers. -->
 
  *  Use named returns to improve readability of function signatures.
 
+ *  When naming a file which defines an enitity, use singular nouns, unless the
+    entity is some form of a container for other entities:
+
+    ```go
+    // File: client.go
+
+    package foo
+
+    type Client struct {
+            // …
+    }
+    ```
+
+    ```go
+    // File: clients.go
+
+    package foo
+
+    type Clients []*Client
+
+    // …
+
+    type ClientsWithCache struct {
+            // …
+    }
+    ```
+
  ###  <a id="testing" href="#testing">Testing</a>
 
  *  Use `assert.NoError` and `require.NoError` instead of `assert.Nil` and