More durable backups

On hosts where long-running PHP processes are killed, backups aren't terribly reliable. We think we can improve their durability by breaking the backup process into smaller pieces.

Here's how it will work:

* [x] Write a `.file-manifest` at the beginning of the backup process.
* [x] Incrementally add files to the archive based on the file manifest; when a file is successfully added, remove it from the manifest.
* [x] In the status file, log the process ID for the backup process. If process ID doesn't exist, return an error that the process was killed or died. The application then can spawn a new process to continue the same backup.
* [x] Report progress (x of x files) in the status file.