Add a "resume failed transfers" function in `zelta sync`

[bellhyve]

The first decision is whether this should be a default mode or if it needs to be enabled. I think it will annoy people either way, but if the promise is a tool that does everything you'd desire for a fast and reliable transfer out of the box, I think "on by default" is the right approach.

[bellhyve]

This is pending a zfs list caching since it's semi-related.