use symlink when possible

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

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

@@ -0,0 +1,24 @@
+#![allow(unused_variables)] // TODO(you): remove this lint after implementing this mod
+#![allow(dead_code)] // TODO(you): remove this lint after implementing this mod
+
+use std::collections::BTreeMap;
+
+pub struct Watermark {
+    readers: BTreeMap<u64, usize>,
+}
+
+impl Watermark {
+    pub fn new() -> Self {
+        Self {
+            readers: BTreeMap::new(),
+        }
+    }
+
+    pub fn add_reader(&mut self, ts: u64) {}
+
+    pub fn remove_reader(&mut self, ts: u64) {}
+
+    pub fn watermark(&self) -> Option<u64> {
+        Some(0)
+    }
+}