* snapshot: support for periodic checkpointing of snapshots in progress
For each snapshot that takes longer than 45 minutes, we trigger
internal cancellation, save the manifest and restart the snapshot
at which point all files will be cached.
This helps ensure the property that no file or directory objects
in the repository remain unreachable from a snapshot root for more than
one hour, which is important from GC perspective.
* nit: unified spelling 'cancelled' => 'canceled'
- cleaned up migration progress output
- fixed migration idempotency
- added migration of policies
- renamed --parallelism to --parallel
- improved e2e test
- do not prompt for password to source repository if persisted
Percentage based on last-known snapshot size
* server: exposed last completed snapshot size in the API
* cli: added support for progress indicator (percentage based on last-known snapshot size)
* htmlui: added progress indicator in the UI (percentage based on last-known snapshot size)
