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
This function isn't used anywhere, so please remove it. Having it there just makes the API more difficult to understand: when do I use one over the other? Is the rootDir really optional? I'd just rename "NewRootedArchive" to "NewArchive" and say that you need a filename and a root-dir.