add progress reporting for restic backups & restores

[skriss]

xref #20 #21

Describe the problem/challenge you have

During long-running restic backups/restores, there is no visibility into what (if anything) is happening, making it hard to know if the backup/restore is making progress or hung, how long the operation might take, etc.

Describe the solution you'd like

We should capture progress during restic operations and make it user-visible somewhere (could be in a status field on the PVB/PVR, or in an Event, or somewhere else - TBD).

restic backup provides progress reporting to stdout. @jmontleon did a POC of capturing this periodically and writing it to the PVB's status (see this comment).

restic restore does not yet have such progress reporting; however, the restic ls SNAPSHOT_ID command will give a listing of all files in a backup, and we could potentially use that along with periodic listing of files in the directory being restored to provide something useful to the user.

This definitely needs some experimentation and design work, but IMHO we need to do this before declaring restic GA, as it's just too hard to know what's going on right now.

@prydonius if you're looking for something to do next week, some R&D here would definitely be useful!

[prydonius]

restic backup provides progress reporting to stdout. @jmontleon did a POC of capturing this periodically and writing it to the PVB's status (see this comment).

FYI took me a while to figure out that the JSON output for restic backup requires 0.9.5, our current images ship 0.9.4, so we'll need to update that.

[skriss]

Closed by #1821 and #1854