fix clippy warnings

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

mini-lsm-starter/src/mvcc/watermark.rs

@@ -21,6 +21,12 @@ pub struct Watermark {
     readers: BTreeMap<u64, usize>,
 }
 
+impl Default for Watermark {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 impl Watermark {
     pub fn new() -> Self {
         Self {