open file remotely without using ssh channel

[Antoine Martin]

The sftp code was removed with paramiko, what we can do to restore the functionality is to define a new message type for file transfers and send the file through the normal client-server channel.

• new accept_files option for servers (defaults to off?)
• keep file pointer + index in server connection
• send first chunk of data, below the max_line limit and also taking into account the link speed
• server creates a temporary file in configurable download location (default to ~/Download)
• server sends an ack after each packet is received (potential small delay here to ensure we prioritize other packets ahead of file transfers)
• client fire progress_update callbacks registered with the file transfer
• client sends the next chunk (potential delay here - same reason as above), taking into account how long the first previous took (so we can keep UI updates regular - say 1 second target)
• if the server (or client) does not ack (or send the next chunk) within a certain amount of time (tbd - dynamic?), we cancel the whole thing and tell the other end. The server can delete the temporary file.
• When the transfer is complete, we ask the server to open it.
• Ensure file data is encrypted over the wire

[Antoine Martin]

• added "allow_file_transfers" option in r3522