Provide a default max_queue_length value to task_type

Because the register task type routine does not provide the value, it's left unchecked. Once a new lister starts listing origins, the scheduler keeps on scheduling new origins in the queue without limits. As the consumption may be slower than the production of messages, that tends towards too much resources usage in rabbitmq.

This should limit the issue for new deployments.

Related to swh/infra/ci-cd/swh-charts!162 (merged)