Cleanup session handling
There is an unused channel system in FD which dates from GOsa and I think was added to attempt to split session vars by browser tab or something like that.
To avoid session problems and make problems easier to debug I suggest we:
- Remove the channel system
- Remove the global_ methods and use always use set/get instead
- Remove the reference returning in session::get. Using a reference on a session var is a bad idea and is most likely the source of some of our problems
- Remove all calls to session_ functions outside of our session class and make sure everything goes through it.