Housekeeping on client connections

[sanity]

Information on WebSocket connection to clients is kept in KWeb.clients, currently a ConcurrentHashMap.

This means that they won't be deleted until the server is restarted. We should keep track of when clients disconnect from the server and delete any associated data.

[sanity]

Per @swishy, Wasabi doesn't yet have a callback for a websocket connection closing, but he is going to add it.

[swishy]

See wasabifx/wasabi#108, thanks Ian.

[sanity]

Note that we also need a mechanism to allow resources to be cleaned up when a client connection is terminated. This could be a callback specified within the buildPage function passed to the KWeb constructor. Here is what it might look like:

KWeb() {
   // ... build page here
  onDisconnect() {
    // Cleanup resources here
  }
}

Here we've added a onDisconnect method to the RootReceiver which registers a listener that is called when this client disconnects. Because it is within the buildPage function it can have access to all variables defined when the page is set up.

[sanity]

This was fixed in commit bb43a39 (sorry about the crappy commit message).