+ 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_file.go

@@ -11,6 +11,10 @@ import (
 
 // flushLogBuffer flushes the current buffer to file and resets the current buffer
 func (l *queryLog) flushLogBuffer(fullFlush bool) error {
+	if !l.conf.FileEnabled {
+		return nil
+	}
+
 	l.fileFlushLock.Lock()
 	defer l.fileFlushLock.Unlock()