message: [accumulator] prettify code
diff: @@ -279,10 +279,7 @@ impl AccumulatorTree {
  }
  fn get_node_index(&self, key: NodeCacheKey) -> Option<HashValue> {
  - match self.index_cache.lock().get(&key) {
  - Some(node_hash) => Some(*node_hash),
  - None => None,
  - }
  + self.index_cache.lock().get(&key).map(|value| *value)
  }
  /// Get node hash always.
  