webrtc: Optimize substream AsyncRead and AsyncWrite

[lexnv]

litep2p/src/transport/webrtc/substream.rs

Lines 191 to 201 in e0b7118

	impl tokio::io::AsyncWrite for Substream {
	fn poll_write(
	self: Pin<&mut Self>,
	cx: &mut Context<'_>,
	buf: &[u8],
	) -> Poll<Result<usize, std::io::Error>> {
	if let State::SendClosed = *self.state.lock() {
	return Poll::Ready(Err(std::io::ErrorKind::BrokenPipe.into()));
	}
	// TODO: try to coalesce multiple calls to `poll_write()` into single `Event::Message`