Method overriden by subclasses to optimize the MultiStream behavior.

def _MultiStream(cls, fds):
    """Method overriden by subclasses to optimize the MultiStream behavior."""
    for fd in fds:
      fd.Seek(0)
      while True:
        chunk = fd.Read(cls.MULTI_STREAM_CHUNK_SIZE)
        if not chunk:
          break
        yield fd, chunk, None