Added REST API endpoints and CLI commands for durable storage management.

New REST endpoints:

- `/ds/sites`
- `/ds/sites/:site`
- `/ds/storages`
- `/ds/storages/:ds`
- `/ds/storages/:ds/replicas`
- `/ds/storages/:ds/replicas/:site`

New CLI commands:

- `ds set_replicas`
- `ds join`
- `ds leave`