Add option to run an `IAMQPWorker` without an `IWorker`

[mathemaphysics]

Sometimes we don't really care about heartbeats in the main operator() loop in IAMQPWorker and we just want to run and ack() right inside it without a separate thread running the job. As mentioned, the downside is that it while you're running the job in the main operator() loop you can't do anything else, including respond to a heartbeat or process any other messages. Those are blocked. The upside is that ack() isn't called as soon as the job is finished.

Refashion the constructor for IAMQPWorker constructor to run without an external worker if the pointer passed in is nullptr.