Add delegate methods to CT::SessionHandler

Let's spruce up CT::SessionHandler with some delegate methods!

With the changes in this pull request, we will be able to do things like

``` ruby
ctdb_session_handler.logout!
ctdb_session_handler.active?
ctdb_session_handler.locked?
```

That's so much better than

``` ruby
ctdb_session_handler.session.logout
ctdb_session_handler.session.active?
ctdb_session_handler.session.locked?
```