kv: Use Bytes as value to avoid copy between read/write

[Xuanwo]

Now, we are store Vec<u8> in all kv adapters. Maybe it's better to store a Bytes instead?

For example, read data from HashMap<String, Bytes> will not need to clone the entire bytes anymore.

[Xuanwo]

@PsiACE, do you wanna to take this issue a look?