checkin initial MVCC codebase

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

mini-lsm-mvcc/src/debug.rs

@@ -0,0 +1,17 @@
+use crate::lsm_storage::MiniLsm;
+
+impl MiniLsm {
+    pub fn dump_structure(&self) {
+        let snapshot = self.inner.state.read();
+        if !snapshot.l0_sstables.is_empty() {
+            println!(
+                "L0 ({}): {:?}",
+                snapshot.l0_sstables.len(),
+                snapshot.l0_sstables,
+            );
+        }
+        for (level, files) in &snapshot.levels {
+            println!("L{level} ({}): {:?}", files.len(), files);
+        }
+    }
+}