Background task manager (#332)

- [x] New background worker trait
- [x] Adapt all current workers to use new API
- [x] Command to list currently running workers, and whether they are active, idle, or dead
- [x] Error reporting
- Optimizations
  - [x] Merkle updater: several items per iteration
  - [ ] Use `tokio::task::spawn_blocking` where appropriate so that CPU-intensive tasks don't block other things going on
- scrub:
  - [x] have only one worker with a channel to start/pause/cancel
  - [x] automatic scrub
  - [x] ability to view and change tranquility from CLI
  - [x] persistence of a few info
- [ ] Testing

Co-authored-by: Alex Auvolat <alex@adnab.me>
Reviewed-on: https://git.deuxfleurs.fr/Deuxfleurs/garage/pulls/332
Co-authored-by: Alex <alex@adnab.me>
Co-committed-by: Alex <alex@adnab.me>

src/rpc/system.rs

@@ -2,7 +2,7 @@
 use std::collections::HashMap;
 use std::io::{Read, Write};
 use std::net::{IpAddr, SocketAddr};
-use std::path::Path;
+use std::path::{Path, PathBuf};
 use std::sync::{Arc, RwLock};
 use std::time::{Duration, Instant};
 
@@ -104,6 +104,9 @@ pub struct System {
 
 	/// The job runner of this node
 	pub background: Arc<BackgroundRunner>,
+
+	/// Path to metadata directory
+	pub metadata_dir: PathBuf,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
@@ -295,6 +298,7 @@ impl System {
 			ring,
 			update_ring: Mutex::new(update_ring),
 			background,
+			metadata_dir: config.metadata_dir.clone(),
 		});
 		sys.system_endpoint.set_handler(sys.clone());
 		sys