message: snapshotter: add method Close() to release database flock
  During testing, several ut cases are exectuing one by one.
  They all need to open their own database, each case should
  close the database once it is done.
diff: @@ -89,6 +89,15 @@ func (d *Database) initDatabase() error {
  })
  }
  +func (d *Database) Close() error {
  + err := d.db.Close()
  + if err != nil {
  + return errors.Wrapf(err, "Close boltdb failed")
  + }
  +
  + return nil
  +}
  +
  // SaveDaemon saves daemon record from database
  func (d *Database) SaveDaemon(ctx context.Context, dmn *daemon.Daemon) error {
  return d.db.Update(func(tx *bolt.Tx) error {
  