Realtime editing doesn't support clustering

If multiple users are editing the same page in real-time in a cluster setup they may end up in different real-time sessions depending on the cluster node they are served by. Basically each cluster node will have its own real-time session. We need to synchronize the real-time session between cluster nodes or make sure that all users are calling the same WebSocket end-point (from one of the cluster nodes).

See https://design.xwiki.org/xwiki/bin/view/Proposal/Clusteringsupportinrealtimeeditor for details on the work in progress.