Since the class needs to be copy-constructible, there may be many
copies of an instance. So instead of writing to the device on every
destructor call, only flush buffer on the last destructor call.
Previously we need a file buffer that is as large as the file size and
this could be a problem when user has less free memory available or
having very large data. Now with the help of `FileOutputIterator`,
we can have a much smaller, fixed size immediate file buffer and also
the code looks nice with `lt::bencode()`.
