patch memtable and add ts for wal

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

mini-lsm-mvcc/src/table/builder.rs

@@ -70,7 +70,9 @@ impl SsTableBuilder {
             first_key: std::mem::take(&mut self.first_key).into_key_bytes(),
             last_key: std::mem::take(&mut self.last_key).into_key_bytes(),
         });
+        let checksum = crc32fast::hash(&encoded_block);
         self.data.extend(encoded_block);
+        self.data.put_u32(checksum);
     }
 
     /// Builds the SSTable and writes it to the given path. Use the `FileObject` structure to manipulate the disk objects.