Safely close database connections and delete backends

Also fix NewClosure leak caused by disconnected object signals

src/tidal/tidalrequest.cpp

@@ -80,7 +80,7 @@ TidalRequest::~TidalRequest() {
 
   while (!album_cover_replies_.isEmpty()) {
     QNetworkReply *reply = album_cover_replies_.takeFirst();
-    disconnect(reply, 0, nullptr, 0);
+    disconnect(reply, 0, this, 0);
     if (reply->isRunning()) reply->abort();
     reply->deleteLater();
   }