+ config: new setting "querylog_file_enabled"

Close #876

Squashed commit of the following:

commit f83f60a7340d8a3f6de7ecfebb426e47d19e83d8
Merge: cfb72869 391e6199
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Thu May 28 15:17:21 2020 +0300

    Merge remote-tracking branch 'origin/master' into 876-logs

commit cfb72869f7cf0bf59a478ab8c7920c273e2fa5f9
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Thu May 28 12:50:02 2020 +0300

    tests

commit 35376e4f450cf66507d733c931b7ed27eff1f36c
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Wed May 27 18:15:12 2020 +0300

    fix

commit 0cfb802d73db52a4b09c459a68a8a18918447b76
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Wed May 27 16:49:52 2020 +0300

    tests

commit 03ca280b6aed3a4880a9d4f4cd18bf47b1c742f6
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Wed May 27 15:32:27 2020 +0300

    + config: new setting "querylog_file_enabled" - query log will be written to a file

querylog/querylog.go

@@ -9,14 +9,6 @@ import (
 	"github.com/miekg/dns"
 )
 
-// DiskConfig - configuration settings that are stored on disk
-type DiskConfig struct {
-	Enabled           bool
-	Interval          uint32
-	MemSize           uint32
-	AnonymizeClientIP bool
-}
-
 // QueryLog - main interface
 type QueryLog interface {
 	Start()
@@ -28,12 +20,13 @@ type QueryLog interface {
 	Add(params AddParams)
 
 	// WriteDiskConfig - write configuration
-	WriteDiskConfig(dc *DiskConfig)
+	WriteDiskConfig(c *Config)
 }
 
 // Config - configuration object
 type Config struct {
-	Enabled           bool
+	Enabled           bool   // enable the module
+	FileEnabled       bool   // write logs to file
 	BaseDir           string // directory where log file is stored
 	Interval          uint32 // interval to rotate logs (in days)
 	MemSize           uint32 // number of entries kept in memory before they are flushed to disk