compute read amp for tiered compaction

Signed-off-by: Alex Chi Z <iskyzh@gmail.com>

mini-lsm/src/compact/tiered.rs

@@ -119,6 +119,9 @@ impl TieredCompactionController {
                 levels.push((output[0], output.to_vec()));
             }
         }
+        if !tier_to_remove.is_empty() {
+            unreachable!("some tiers not found??");
+        }
         snapshot.levels = levels;
         (snapshot, files_to_remove)
     }