bug #4106 : Convert SessionManager and SilverStatisticsManager into Spring-managed bean. 

The life-cycles of the session management service and of the statistics management services are now managed by the IoC container (yet Spring). So, the dependencies on other Spring-managed beans can be set in a more visible way: the session management implementation depends on the statistics management service and on the scheduling API, and the statistics management service depends on the notification API.

In order to remove any direct use of the service implementation (SessionManager), some additional methods are defined in the session management interface so that the session management is only used by its interface.

By letting the life-cycles of the session and statistics management beans made by the IoC container, the initialization and the shutdown of theses services can be directly operated through the IoC container, thus within a well-living web container in which all the services (like JNDI) are actives.