Twisted service to manage polling a DirectoryQueue for new entries.
DirectoryQueueService is a Twisted IService that polls a DirectoryQueue and fires callbacks to process new queue items when they become available.
The DirectoryQueueService supports processing items concurrently, up to the concurrency level specified by the constructor parameter.
Note that the user-supplied callbacks must take care of the details of running asynchronously for actual concurrency to occur. See itemReadyCallback method.