SftpClient: DowloadFile does not perform well on Sun SSH

[drieseng]

While I'm seeing throughput increases of up to 50% when used with OpenSSH, the switch to a read-ahead mechanism actually regressed the throughput with Sun SSH (1.1.5).

There appear to be two causes for this:

• Sun's SSH implementation does not seem to enjoy receiving a SSH_FXP_OPEN and a SSH_FXP_LSTAT request for a given file at the same time.
• We performing async read-aheads until we've received a zero-length response (signaling EOF).
  This means that we risk having a few SSH_FXP_READ requests with an offset beyond EOF.
  This causes Sun SSH to take a brief moment of reflection.