Fix resource leak due to Files.walk

Stream creates by File.walk should be closed, like jdk said:

• <p> The returned stream encapsulates one or more {@link DirectoryStream}s.
• If timely disposal of file system resources is required, the
• {@code try}

  -with-resources construct should be used to ensure that the

• stream's {@link Stream#close close} method is invoked after the stream
• operations are completed. Operating on a closed stream will result in an
• {@link java.lang.IllegalStateException}

  .