finish week 1 day 6

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

mini-lsm-starter/src/iterators.rs

@@ -13,4 +13,9 @@ pub trait StorageIterator {
 
     /// Move to the next position.
     fn next(&mut self) -> anyhow::Result<()>;
+
+    /// Number of underlying active iterators for this iterator.
+    fn num_active_iterators(&self) -> usize {
+        1
+    }
 }