SQS consumers run independently of the AwsSqsJobHandlerSubscriptionService that spawned them

[Josh Matthews]

AwsSqsJobHandlerSubscriptionService's startup method calls consumer.subscribe for all known consumer mappings. Its shutdown method is empty, however, while each subscribe call launches a repeated task queue task that asynchronously fetches job queue data and processes it. This means that the AwsSqsJobHandlerSubscriptionService reports it has shut down immediately, while the actual tasks doing work continue to do so, leading to spurious errors if the jobs depend on other services that get shut down in the meantime.