implement zerocopy read/write on Stream with splice

## What is the problem your feature solves, or the need it fulfills?

I would like to forward traffic from downstream to upstream in an L4 proxy using splice to avoid multiple user-space memory copies. However, I've found it difficult to implement this using the Stream object. Is there any way to achieve this?

## Describe the solution you'd like

maybe expose the underlying tcpstream from the Stream object?, appreciate if someone can shed some light on it