simple sketch of libwebsocket_write/LWS_CALLBACK_CLIENT_RECEIVE example

1. use LWS_CALLBACK_CLIENT_RECEIVE listen recv
2. use libwebsocket_write write data
3. data buffer contains [LWS_SEND_BUFFER_PRE_PADDING,
LWS_SEND_BUFFER_POST_PADDING]
4. use libwebsocket_callback_on_writable to tip ready to write