Snapshot backups
This MR adds support for snapshot backups to the renter, including API endpoints.
To keep things simple, all transfers happen serially (one host at a time), and simple replication is used (no erasure coding).
The next major step is to add snapshot loading functionality. We can already download snapshots, but we don't have a method that will load that snapshot into the renter, nor an API endpoint to trigger a load.
Edited by Luke Champine