Pull request: * querylog: fix end of log handling

Merge in DNS/adguard-home from 2229-query-log to master

Closes #2229.

Squashed commit of the following:

commit 32508a3f3b1e098869e1649a2774f1f17d14d41f
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Nov 10 16:51:25 2020 +0300

    * querylog: add test

commit 774159cc313a0284a8bb8327489671e5d7a3e4eb
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Nov 10 15:26:26 2020 +0300

    * querylog: better errors

commit 27b13a4dcaff9e8f9b08aec81c0c03f62ebd3fa5
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Nov 10 15:18:51 2020 +0300

    * querylog: fix end of log handling

internal/querylog/qlog_reader.go

@@ -1,6 +1,7 @@
 package querylog
 
 import (
+	"errors"
 	"io"
 
 	"github.com/AdguardTeam/AdGuardHome/internal/agherr"
@@ -52,11 +53,14 @@ func (r *QLogReader) Seek(timestamp int64) error {
 	for i := len(r.qFiles) - 1; i >= 0; i-- {
 		q := r.qFiles[i]
 		_, _, err := q.Seek(timestamp)
-		if err == nil {
-			// Our search is finished, we found the element we were looking for
-			// Update currentFile only, position is already set properly in the QLogFile
+		if err == nil || errors.Is(err, ErrEndOfLog) {
+			// Our search is finished, and we either found the
+			// element we were looking for or reached the end of the
+			// log.  Update currentFile only, position is already
+			// set properly in QLogFile.
 			r.currentFile = i
-			return nil
+
+			return err
 		}
 	}