[Fence Extensions] Support ClientFence and ServerFence

To properly support switching pixel formats during an active connection we should implement [ClientFence](https://github.com/rfbproto/rfbproto/blob/master/rfbproto.rst#clientfence) and [ServerFence](https://github.com/rfbproto/rfbproto/blob/master/rfbproto.rst#serverfence).