Replace the legacy backups implementation with a new one.

The new implementation is refactored to fit to the new backups approach of storing the backups in env storage (along with metadata in mongo). This is the final large patch in the backup series (the remaining ones are much smaller).

The next 2 patches in the series:
https://github.com/ericsnowcurrently/juju/pull/3
https://github.com/ericsnowcurrently/juju/pull/4
Yeah, I took a stab at it in a different PR and it was messy, so I just hard-coded them here and left the note.  It's definitely something to tackle later.  Thanks for mentioning juju/paths.  That's along the lines of what I was hoping to take advantage.