Both the concurrent collection classes DelayQueue and PriorityBlockingQueue
have iterators that will throw ConcurrentModificationException in the
presence of concurrent modification. This is unreasonable.
Concurrent collections should be concurrently iterable and modifyable.

It appears that this is due to implementor laziness.
A backing non-concurrent PriorityQueue is used, and delegating iteration
to the PriorityQueue iterator leads naturally to ConcurrentModificationException.