storage: use slice as variable for key_with_epoch

[Little-Wallace]

Is your feature request related to a problem? Please describe.

If we pass an variable Vec<u8> we must re-allocate this key if the capacity of this vector is less than the sum length of user_key and epoch.
In most where we must convert slice to Vec and then pass it to key_with_epoch, and it means that we would allocate memory twice.

Describe the solution you'd like

It is better to pass a slice and allocate the whole key in method key_with_epoch.

fn key_with_epoch(user_key: &[u8], epoch: HummockEpoch) -> Vec<u8> {

}

Describe alternatives you've considered

No response

Additional context

No response

[BugenZhao]

FYI, we may use Buf::chain if we really care about the allocation and copying. 🤔
https://docs.rs/bytes/latest/bytes/trait.Buf.html#method.chain

[Little-Wallace]

It seems to be complex...

[Little-Wallace]

FYI, we may use Buf::chain if we really care about the allocation and copying. 🤔 https://docs.rs/bytes/latest/bytes/trait.Buf.html#method.chain

key_with_epoch is also not convenient for unit test

[hzxa21]

After #6130, key_with_epoch is only used in tests so I think we can close this issue.