refactoring

refactoring

This is groundwork for using git-annex transferkeys to run transfers,
in order to allow stalled transfers to be interrupted and retried.

The new upload and download are closer to what git-annex transferkeys
does, so the plan is to make them use it.

Then things that were left using upload' and download' won't recover
from stalls. Notably, that includes import and export. But
at least get/move/copy will be able to. (Also the assistant hopefully,
but not yet.)

This commit was sponsored by Jake Vosloo on Patreon.