[space_map (rust)] Fix space map building

- Fix out-of-bounds index
- Automatically flush queued writes before function return
- Track allocated blocks in write_batcher (might be space consuming)

src/write_batcher.rs

@@ -43,6 +43,7 @@ impl WriteBatcher {
             return Err(anyhow!("out of metadata space"));
         }
 
+        self.allocations.insert(b.unwrap());
         Ok(Block::new(b.unwrap()))
     }