allow starting separate api and workers in different processes

This is useful for local debugging, because it allows to start separate processes:

• one process that listens on port 8040 (api) but without a local orchestration_worker (RABBITMQ_WORKER_PREFETCH_COUNT=0).
• several other processes that listen on other ports with orchestration_workers.

That allows debugging situations where we can SIGTERM an actively running orchestration_worker and see how that unfinished work is re-queued to the front of the rabbitmq orchestrator queue and then reassigned to another worker...