Fix authentication corner case.

When role changes in a new authentication attempt.
Was missing a method on MainLoop instance to unregister all plugins.

ccnode/node.py

@@ -286,6 +286,11 @@ class MainLoop(object):
 
         plugin.stop()
 
+    def close_plugins(self):
+        """Unregister all plugins from the loop."""
+        for plugin in self.registered_plugins.copy():
+            self.unregister_plugin(plugin)
+
     def restart_rpc_connection(self, *args):
         if not self.rpc_connected:
             return