Fix tungstenite buffering parameters for uart WS

[pfmooney]

When tungstenite was updated to 0.20.x, the structure of the buffering parameters exposed by its websocket configuration changed significantly. The parameters chosen can cause WriteBufferFull errors if enough output accumulates in the history buffer to be sent in a message which exceeds the unnecessarily small max_write_buffer_size.

Using the configuration defaults, which feature an unbounded max_write_buffer_size, should provide acceptable behavior.

Fixes #540

[leftwo]

Yeah, the usize::MAX is probably large enough.

impl Default for WebSocketConfig {
    fn default() -> Self {
        #[allow(deprecated)]
        WebSocketConfig {
            max_send_queue: None,
            write_buffer_size: 128 * 1024,
            max_write_buffer_size: usize::MAX,
            max_message_size: Some(64 << 20),
            max_frame_size: Some(16 << 20),
            accept_unmasked_frames: false,
        }
    }
}