Fix #1684 WebSocket proxy behind a buffered proxy.

Fix #1684 WebSocket proxy behind a buffered proxy.

The given patch fixes incoming WebSocket traffic behind buffered Proxies
like NGINX.

NGINX buffers multiple requests from the Browser into one frame and sends that to pveproxy,
before this patch we then processed the first message of the frame and cleared the buffer which
may contained more messages.
With this patch we process each message and clear the buffer right.

This fixes the "NoVNC blank screen" problem users reported on the forums.