checkin part 2 solution

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

mini-lsm-starter/src/mem_table.rs

@@ -88,6 +88,11 @@ impl MemTable {
         self.approximate_size
             .load(std::sync::atomic::Ordering::Relaxed)
     }
+
+    /// Only use this function when closing the database
+    pub fn is_empty(&self) -> bool {
+        self.map.is_empty()
+    }
 }
 
 type SkipMapRangeIter<'a> =