[block-ref-repair] Block refcount recalculation and repair

- We always recalculate the reference count of a block before deleting
  it locally, to make sure that it is indeed zero.

- If we had to fetch a remote block but we were not able to get it,
  check that refcount is indeed > 0.

- Repair procedure that checks everything

src/model/garage.rs

@@ -247,6 +247,14 @@ impl Garage {
 		#[cfg(feature = "k2v")]
 		let k2v = GarageK2V::new(system.clone(), &db, meta_rep_param);
 
+		// ---- setup block refcount recalculation ----
+		// this function can be used to fix inconsistencies in the RC table
+		block_manager.set_recalc_rc(vec![
+			block_ref_recount_fn(&block_ref_table),
+			// other functions could be added here if we had other tables
+			// that hold references to data blocks
+		]);
+
 		// -- done --
 		Ok(Arc::new(Self {
 			config,