session pollution on worker calls

Each call to worker/queue and each PUT to a concrete worker on starting it produces new session DB entries.
This leads to a heavy pollution of our session table which affects the overall performance.
We decided to delete the session Entries for the worker Controller. Currently this happens by setting the modified time to 0 in the workerController __destruct.
This has the negative effect that a session entry is written to the db, and will be afterwards deleted because auf the timeout of 0. This can be better if we dont allow the session to be saved in the DB before by destroying it before the session_save handler is called.
See therefore the TODO comments in the WorkerController.