lib/model, lib/stats: Keep track of folder's last scan time (ref #3143)

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3250
LGTM: calmh, AudriusButkevicius

lib/model/model.go

@@ -1602,6 +1602,7 @@ func (m *Model) internalScanFolderSubdirs(folder string, subs []string) error {
 		m.updateLocalsFromScanning(folder, batch)
 	}
 
+	m.folderStatRef(folder).ScanCompleted()
 	runner.setState(FolderIdle)
 	return nil
 }