implement mvcc compaction + snapshot

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

mini-lsm/src/lsm_storage.rs

@@ -256,6 +256,10 @@ impl MiniLsm {
         self.inner.sync()
     }
 
+    pub fn new_txn(&self) -> Result<()> {
+        self.inner.new_txn()
+    }
+
     pub fn scan(
         &self,
         lower: Bound<&[u8]>,
@@ -668,6 +672,11 @@ impl LsmStorageInner {
         Ok(())
     }
 
+    pub fn new_txn(&self) -> Result<()> {
+        // no-op
+        Ok(())
+    }
+
     /// Create an iterator over a range of keys.
     pub fn scan(
         &self,