Revise "save resume data" handling on shutdown

When shutting down, instead of waiting for all types of alert from
libtorrent, now it only waits for specific alert types. This potentially
help shorten the shutdown waiting time.

src/base/bittorrent/sessionimpl.cpp

@@ -2892,28 +2892,41 @@ void SessionImpl::saveResumeData()
         ++m_numResumeData;
     }
 
+    QElapsedTimer timer;
+    timer.start();
+
     while (m_numResumeData > 0)
     {
-        const std::vector<lt::alert *> alerts = getPendingAlerts(lt::seconds {30});
-        if (alerts.empty())
-        {
-            LogMsg(tr("Aborted saving resume data. Number of outstanding torrents: %1").arg(QString::number(m_numResumeData))
-                , Log::CRITICAL);
-            break;
-        }
+        const lt::seconds waitTime {5};
+        const lt::seconds expireTime {30};
+        const std::vector<lt::alert *> alerts = getPendingAlerts(waitTime);
 
+        bool hasWantedAlert = false;
         for (const lt::alert *a : alerts)
         {
             switch (a->type())
             {
             case lt::save_resume_data_failed_alert::alert_type:
+                hasWantedAlert = true;
                 --m_numResumeData;
                 break;
             case lt::save_resume_data_alert::alert_type:
+                hasWantedAlert = true;
                 dispatchTorrentAlert(static_cast<const lt::torrent_alert *>(a));
                 break;
             }
         }
+
+        if (hasWantedAlert)
+        {
+            timer.start();
+        }
+        else if (timer.hasExpired(lt::total_milliseconds(expireTime)))
+        {
+            LogMsg(tr("Aborted saving resume data. Number of outstanding torrents: %1").arg(QString::number(m_numResumeData))
+                , Log::CRITICAL);
+            break;
+        }
     }
 }