implement mvcc compaction + snapshot

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

mini-lsm-starter/src/lsm_storage.rs

@@ -162,6 +162,10 @@ impl MiniLsm {
         }))
     }
 
+    pub fn new_txn(&self) -> Result<()> {
+        self.inner.new_txn()
+    }
+
     pub fn write_batch<T: AsRef<[u8]>>(&self, batch: &[WriteBatchRecord<T>]) -> Result<()> {
         self.inner.write_batch(batch)
     }
@@ -300,6 +304,11 @@ impl LsmStorageInner {
         unimplemented!()
     }
 
+    pub fn new_txn(&self) -> Result<()> {
+        // no-op
+        Ok(())
+    }
+
     /// Create an iterator over a range of keys.
     pub fn scan(
         &self,