LSP: use asynchronous shutdown for running servers

Change-Id: I3b1853177d07c98e051de0bc9f494389fbf4c104
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/languageclient/languageclientplugin.h

@@ -43,6 +43,7 @@ public:
 private:
     bool initialize(const QStringList &arguments, QString *errorString) override;
     void extensionsInitialized() override;
+    ShutdownFlag aboutToShutdown() override;
 
 private:
     LanguageClientManager m_clientManager;