checkin initial MVCC codebase

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

mini-lsm-mvcc/src/iterators.rs

@@ -0,0 +1,26 @@
+pub mod concat_iterator;
+pub mod merge_iterator;
+pub mod two_merge_iterator;
+
+pub trait StorageIterator {
+    type KeyType<'a>: PartialEq + Eq + PartialOrd + Ord
+    where
+        Self: 'a;
+
+    /// Get the current value.
+    fn value(&self) -> &[u8];
+
+    /// Get the current key.
+    fn key(&self) -> Self::KeyType<'_>;
+
+    /// Check if the current iterator is valid.
+    fn is_valid(&self) -> bool;
+
+    /// Move to the next position.
+    fn next(&mut self) -> anyhow::Result<()>;
+
+    /// Number of underlying active iterators for this iterator.
+    fn num_active_iterators(&self) -> usize {
+        1
+    }
+}