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
Don't need to do it in this PR (already big enough!), but there's github.com/juju/juju/juju/paths for this now. Since the set of files is being formed on the machine doing the backup, you can just use `version.Current.Series` in the function parameters.