fix: fix spawn split

[driftluo]

When is_readable is true, it will decode from the read buffer.
https://github.com/tokio-rs/tokio/blob/master/tokio-util/src/codec/framed_impl.rs#L183

But FramedRead::new always sets is_readable to false. So, read buffer replace will never be used unless it reads the second message from StreamHandle.

Now, let's build the split differently, preserving its internal state.