Bug 797906 - GnuCash data file is locked after a "save-as"...

to mysql database.

It's more general, any save-as to a different backend type, i.e.
xml->sql or sql->xml, left the previous book locked.

Ensure that the session is ended and the book unlocked in the
backend's destructor. This also fixes a memory leak in the SQL
backend because the GncSqlConnection wasn't being deleted.

libgnucash/backend/xml/gnc-xml-backend.cpp

@@ -49,6 +49,11 @@ extern "C"
 #define FILE_URI_PREFIX "file://"
 static QofLogModule log_module = GNC_MOD_BACKEND;
 
+GncXmlBackend::~GncXmlBackend()
+{
+    session_end();
+};
+
 bool
 GncXmlBackend::check_path (const char* fullpath, bool create)
 {

libgnucash/backend/xml/gnc-xml-backend.hpp

@@ -34,7 +34,7 @@ public:
     GncXmlBackend operator=(const GncXmlBackend&) = delete;
     GncXmlBackend(const GncXmlBackend&&) = delete;
     GncXmlBackend operator=(const GncXmlBackend&&) = delete;
-    ~GncXmlBackend() = default;
+    ~GncXmlBackend();
     void session_begin(QofSession* session, const char* new_uri,
                        SessionOpenMode mode) override;
     void session_end() override;