v2.0.1: exit cleanly if asio context is re-used by plugin

When the streamdeck software is exiting, it closes the websocket connections to plugins; this usually stops the asio event loop, but if there are pending events (which is possible if a plugin uses GetAsioContext()), it will not exit cleanly, and eventually be killed by the StreamDeck software.

This release explicitly shuts down the event loop when the websocket connection is closed.