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/db/sqlite_adapter.rs

@@ -6,7 +6,7 @@ use std::pin::Pin;
 use std::ptr::NonNull;
 use std::sync::{Arc, Mutex, MutexGuard};
 
-use log::trace;
+use tracing::trace;
 
 use rusqlite::{params, Connection, Rows, Statement, Transaction};