patch memtable and add ts for wal

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

mini-lsm-starter/src/mem_table.rs

@@ -51,6 +51,22 @@ impl MemTable {
         unimplemented!()
     }
 
+    pub fn for_testing_put_slice(&self, key: &[u8], value: &[u8]) -> Result<()> {
+        self.put(key, value)
+    }
+
+    pub fn for_testing_get_slice(&self, key: &[u8]) -> Option<Bytes> {
+        self.get(key)
+    }
+
+    pub fn for_testing_scan_slice(
+        &self,
+        lower: Bound<&[u8]>,
+        upper: Bound<&[u8]>,
+    ) -> MemTableIterator {
+        self.scan(lower, upper)
+    }
+
     /// Get a value by key.
     pub fn get(&self, _key: &[u8]) -> Option<Bytes> {
         unimplemented!()