Right now a queue's configuration is spread across multiple keys in the config. We should move to something like this instead and stop using the separate settings ONLY_QUEUES, BATCH_QUEUES, SINGLE_WORKER_QUEUES/MAX_WORKERS_PER_QUEUE?

QUEUES = {
 'a': {
 max_workers: 5,
 batch: 50
 },
 'b': {}
}

The SINGLE_WORKER_QUEUES setting should be deprecated since MAX_WORKERS_PER_QUEUE=1 is equivalent. Max workers was added in #115.