Closing the Outbox is now working with interrupts.
Details: The bugfix in commit e33b0900dab1e9d5e635f2b911b2e12052c4791b raised a new (much harder) bug. The close()-method never terminates because worker.join() blocks until the worker thread has closed. The worker thread itself was blocking in outbox.take() and did not receive the signal passed over a boolean flag. Deadlock.