pub fn put(&self, key: KeySlice, value: &[u8]) -> Result<()> {
        self.put_batch(&[(key, value)])
    }
Put a key-value pair into the mem-table.

In week 1, day 1, simply put the key-value pair into the skipmap.
In week 2, day 6, also flush the data to WAL.
In week 3, day 5, modify the function to use the batch API.
mini-lsm-mvcc/src/mem_table.rs