Make it easy for WebSocket end-points to support clustering

The WebSocket end-points, unlike the REST end-points, are statefull which means that in a cluster setup this state may need to be synchronized across all cluster members (XWiki servers). Not all WebSocket end-points need this, but some do, like the Netflux end-point used by the real-time editing. We need to see if we can provide some generic helpers to ease the implementation of cluster support in WebSocket end-points.