Managed backups: restore and download

v0.8.0-beta

Managed backups: restore and download. The agent can now (1) produce a backup of the database of an app attached to its project (path and size reported to the hub), (2) restore a backup on demand (reapplying the dump, then restarting the app), and (3) stream a backup archive to the hub on demand, to allow downloading it from the dashboard without intermediate storage. Restoring is a destructive operation serialised per app; the hub takes a safety backup just before.