New way to handle local backups.

Now creates a local backup mechanism, where the backend runs within the local client, rather than awkwardly creating a separate process.
Note that if your /etc/tardis/system.defaults, job configurations, environment variables, and switches create a situation where you've specified both a local backup and a remote, you may need to specify --local or --remote.
Also, reworked the directory walker to use os.scandir() instead of os.listdir(), and generally improve tree walking performance by nearly half (in some cases).
Use with caution.