Major Description
When running multiple producers and a single consumer on a queue, the unblock code NPEs when trying to remove a queue entry, from the _blockingQueues Map, that is not present. This is due to the Map being given a Boolean keyed by Queue and in the case above the Map.remove() method returns null.
The data structures used to hold the blocking queues and sessions used in SimpleAMQQueue and the AMQSessionModel implementations (ServerSession & AMQCHannel) should be changed from ConcurrentMaps to Sets - specifically ConcurrentSkipListSet. This will prevent the NPE from being possible.