improve internal item counter mechanisms and implement bucket quotas (#326)

- [x] Refactoring of internal counting API
- [x] Repair procedure for counters (it's an offline procedure!!!)
- [x] New counter for objects in buckets
- [x] Add quotas to buckets struct
- [x] Add CLI to manage bucket quotas
- [x] Add admin API to manage bucket quotas
- [x] Apply quotas by adding checks on put operations
- [x] Proof-read

Co-authored-by: Alex Auvolat <alex@adnab.me>
Reviewed-on: https://git.deuxfleurs.fr/Deuxfleurs/garage/pulls/326
Co-authored-by: Alex <alex@adnab.me>
Co-committed-by: Alex <alex@adnab.me>

src/db/lmdb_adapter.rs

@@ -139,6 +139,14 @@ impl IDb for LmdbDb {
 		Ok(old_val)
 	}
 
+	fn clear(&self, tree: usize) -> Result<()> {
+		let tree = self.get_tree(tree)?;
+		let mut tx = self.db.write_txn()?;
+		tree.clear(&mut tx)?;
+		tx.commit()?;
+		Ok(())
+	}
+
 	fn iter(&self, tree: usize) -> Result<ValueIter<'_>> {
 		let tree = self.get_tree(tree)?;
 		let tx = self.db.read_txn()?;