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/cppsnapshotupdater.cpp

@@ -41,6 +41,7 @@ SnapshotUpdater::SnapshotUpdater(const QString &fileInEditor)
     , m_fileInEditor(fileInEditor)
     , m_editorDefinesChangedSinceLastUpdate(false)
     , m_usePrecompiledHeaders(false)
+    , m_forceSnapshotInvalidation(false)
 {
 }
 
@@ -62,6 +63,11 @@ void SnapshotUpdater::update(CppModelManager::WorkingCopy workingCopy)
 
     updateProjectPart();
 
+    if (m_forceSnapshotInvalidation) {
+        invalidateSnapshot = true;
+        m_forceSnapshotInvalidation = false;
+    }
+
     if (m_projectPart) {
         configFile += m_projectPart->defines;
         includePaths = m_projectPart->includePaths;
@@ -186,6 +192,14 @@ void SnapshotUpdater::update(CppModelManager::WorkingCopy workingCopy)
     }
 }
 
+void SnapshotUpdater::releaseSnapshot()
+{
+    QMutexLocker locker(&m_mutex);
+    m_snapshot = Snapshot();
+    m_deps = DependencyTable();
+    m_forceSnapshotInvalidation = true;
+}
+
 Document::Ptr SnapshotUpdater::document() const
 {
     QMutexLocker locker(&m_mutex);