refactor: Refactor oio::Write into poll-based to create more room for optimization

[Xuanwo]

oio::Write used to be

#[async_trait]
trait Write {
  async fn write(&mut self, bs: &dyn oio::WriteBuf) -> Result<usize>;
}

It works, but the total overhead is high. For example:

• Every call to write needs to build a new BoxFuture<'async_trait, X> (because of async_trait)
• Implement AsyncWrite requires copy the whole buffer every time.

// FIXME: This will copy the buf everytime, we should avoid this.
let bs = Bytes::copy_from_slice(buf);
let fut = async move {
    let n = w.write(&bs).await?;
    Ok((n, w))
};
self.state = State::Write(Box::pin(fut));

By changing into poll_based, we can only build futures while needed. For services like fs, hdfs, we can poll the writer directly! This change also improves a lot for our buffer logic.

[suyanhanx]

Thanks

[silver-ymz]

This should be "SftpWriter doesn't support abort"

[Xuanwo]

This should be "SftpWriter doesn't support abort"

Oh, my bad. Would you like to help fix this typo?