Allow transfer via ncat rather than SSH

SSH can be very slow on very slow hardware (duh). Ncat is very easy to work with and could allow massive performance improvement. The ncat server could even be initiated via ssh.

The issue names no files, tests, or entry points. First trace how remote btrfs backups currently use SSH, then determine the scope for starting an ncat server through SSH and transferring data over it; done would require a defined, working alternative with a measurable performance benefit.

Written by the indexing model from the issue text.