implement mvcc compaction + snapshot

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

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

@@ -0,0 +1,29 @@
+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) {
+        *self.readers.entry(ts).or_default() += 1;
+    }
+
+    pub fn remove_reader(&mut self, ts: u64) {
+        let cnt = self.readers.get_mut(&ts).unwrap();
+        *cnt -= 1;
+        if *cnt == 0 {
+            self.readers.remove(&ts);
+        }
+    }
+
+    pub fn watermark(&self) -> Option<u64> {
+        self.readers.first_key_value().map(|(ts, _)| *ts)
+    }
+}