Before this change we released the ssh connection back to the pool
before the upload was finished.
This meant that uploads were re-using the same ssh connection which
reduces throughput.
This releases the ssh connection back to the pool only after the
upload has finished, or on error state.
See: https://forum.rclone.org/t/sftp-backend-opens-less-connection-than-expected/40245
3ed4a2e963