C++: Release more documents.

- fix memory leak in find-usages
- do not retain snapshot in search history
- when an editor is invisible for more than 2 minutes, release the
  backing snapshot

Retaining snapshots will retain their documents, and if done for too
long, the memory consumption might grow. This is especially the case
when switching to a different kit (Qt version): in that case, the new
versions of headers will be indexed, while the old ones stay around.

Task-number: QTCREATORBUG-5583
Task-number: QTCREATORBUG-7645
Task-number: QTCREATORBUG-9842

Change-Id: I045eda1565e0a3fa702baeffaab9c12662f90289
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>

src/plugins/cpptools/cppfindreferences.h

@@ -78,7 +78,10 @@ public:
 
     CPlusPlus::DependencyTable updateDependencyTable(CPlusPlus::Snapshot snapshot);
 
-private Q_SLOTS:
+public slots:
+    void flushDependencyTable();
+
+private slots:
     void displayResults(int first, int last);
     void searchFinished();
     void cancel();
@@ -101,7 +104,7 @@ private:
                     const CPlusPlus::Snapshot &snapshot, CPlusPlus::LookupContext *context);
 
 private:
-    QPointer<CppModelManagerInterface> _modelManager;
+    QPointer<CppModelManagerInterface> m_modelManager;
     QMap<QFutureWatcher<CPlusPlus::Usage> *, QPointer<Find::SearchResult> > m_watchers;
 
     mutable QMutex m_depsLock;