Introduce unencoded WriteBatch

[Fullstop000]

Currently, the WriteBatch follows the original implementation of leveldb, which is a Vec<u8> with encoded entries. When we call put, the entry will be encoded into variant length based encoded bytes, fill the bytes into a WriteBatch and then be inserted into MemTable, However, when inserting the batch into MemTable, it needs to be decoded into entires first.

For a more efficient inserting style, we might just let WriteBatch hold unencoded entries, and deliver them to the MemTable without any serde.