Add Connection::params() to access parameters passed in handshake request

[thekid]

 class Chat extends Listener {

   public function open($connection) {
  
-    // FIXME: Connection should have path and params separately
-    $path= $connection->path();
-    parse_str(substr($path, strpos($path, '?') + 1), $params);
  
-    if ($for= $params['for'] ?? null) {
+    if ($for= $connection->param('for')) {
       $this->clients->subscribe($connection->id(), $for);
     }
   }

   // ...
 }