Cleanup session handling

Some crash we had recently seems related to sessions ( #6003 #6023 ).

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:

1. Remove the channel system
2. Remove the global_ methods and use always use set/get instead
3. 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
4. Remove all calls to session_ functions outside of our session class and make sure everything goes through it.