Move Protocol response (de)serialization into separate thread

The Protocol DataStream uses waitForData() for blocking wait which makes it
unsuitable for use from the main thread. Also the Protocol is now so fast, that
it blocks GUI when opening larger folders simply because we constantly have data
to deserialize and we don't return to event loop often enough. Moving this
to a separate thread is the right thing to do. It also notably speeds up opening
larger folders.