Improve write performance with larger COPY_CHUNK_SIZE

[alamaison]

We can copy files to server faster with a bigger COPY_CHUNK_SIZE (and matching stream buffer size). However, the size needs to be small for smaller files and slow connections so that progress is updated frequently and so the user can cancel the transfer.

The solution is to manage the chunk size adaptively. Start off small. If the transfer is timely, increase the size to something proportional to file size.