Allow the use of alternative queues

[mjordan]

The built-in queue subsystem used a local CSV file. This is sufficient for most uses but hasn't been tested for large-scale queues. It would be useful to see how it performs compared to standard queue/message brokers. Symfony provides support (via https://symfony.com/doc/4.4/messenger.html) for AMQP/RabbitMQ, Doctrine, and Redis.