Cleanly shutdown the actor
It should be possible to cleanly shutdown the Actor without leaking any resources. This likely means that we have to free an potentially blocked Context within the Actor if we shut the Actor down.
The question is also if an Actor, that is in state "PerformingShutdown", should process remaining queue items (and we only close the input side of the queue), or if close right away (and potentially return the remaining queue item to the closing entity).