input/Tidal*Request: add Start() method

The caller should be able to safely store the instance pointer before
his callback is invoked.

src/input/plugins/TidalInputPlugin.cxx

@@ -91,6 +91,7 @@ TidalInputStream::OnTidalSession() noexcept
 								    tidal_session->GetSession().c_str(),
 								    track_id.c_str(),
 								    handler);
+		track_request->Start();
 	} catch (...) {
 		Failed(std::current_exception());
 	}