Fix handling non trivial stream reads in async framer

[mmcomando]

This fixes cases:

• Stream returns only part of a websocket frame
• Stream returns only part of websocket frame header
• There is no way to tell if framer can progress without reading from stream

In my embedded app I have found out that using async framer doesn't work, when a lot of data is sent.
Async framer works only if stream read returns only one complete frame.
If stream returns to little or too much data then I had problems with framer.

There is also a problem when stream returns only part of websocket frame header, in that case framer can't progress further.

I don't have much experience with async rust streams so maybe this can be done better.

Code in my app which works after this patch:

async fn handle_websocket_reads_and_writes<'a>(
    stream: &mut TcpSocketStream<'a>,
    framer: &mut Framer<embedded_websocket::EmptyRng, embedded_websocket::Server>,
    frame_buf: &mut [u8],
) -> Result<(), ()> {
    loop {
        match select(
            // wait_read_ready should be cancel safe, framer is not
            stream.socket.wait_read_ready(),
            SEND_MESSAGES.receive(),
        )
        .await
        {
            Either::First(_) => {
                loop {
                    let read_result = framer.read(stream, frame_buf).await;
                    handle_websocket_incoming_msg(stream, framer, read_result).await?;

                    // Stream socket might be finished but framer might have data left in it.
                    // Read from framer until it doesn't have buffered data.
                    if !framer.read_ready() {
                        break;
                    }
                }
                return Ok(());
            }
            Either::Second(msg) => {
                send_websocket_message(stream, framer, &msg).await?;
            }
        }
    }
}