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>

Makefile

@@ -1,13 +1,27 @@
-.PHONY: doc all release shell
+.PHONY: doc all release shell run1 run2 run3
 
 all:
 	clear; cargo build --all-features
 
-doc:
-	cd doc/book; mdbook build
-
 release:
 	nix-build --arg release true
 
 shell:
 	nix-shell
+
+# ----
+
+run1:
+	RUST_LOG=garage=debug ./target/debug/garage -c tmp/config1.toml server
+run1rel:
+	RUST_LOG=garage=debug ./target/release/garage -c tmp/config1.toml server
+
+run2:
+	RUST_LOG=garage=debug ./target/debug/garage -c tmp/config2.toml server
+run2rel:
+	RUST_LOG=garage=debug ./target/release/garage -c tmp/config2.toml server
+
+run3:
+	RUST_LOG=garage=debug ./target/debug/garage -c tmp/config3.toml server
+run3rel:
+	RUST_LOG=garage=debug ./target/release/garage -c tmp/config3.toml server