close previous db-load job

And not rely on job destructor, it seems that Qt may postpone
object deletion after deleteLater() invocation for very long time.
In such situation data are still loading and job is holding
database read-lock.

libosmscout-client-qt/src/osmscoutclientqt/PlaneMapRenderer.cpp

@@ -474,7 +474,8 @@ void PlaneMapRenderer::TriggerMapRendering(const MapViewStruct& request, size_t
   {
     QMutexLocker locker(&lock);
     if (loadJob!=nullptr){
-      // TODO: check if job contains same tiles...
+      // TODO: check if job contains the same tiles...
+      loadJob->Close();
       loadJob->deleteLater();
       loadJob=nullptr;
     }