Bug 32481: Limit prefetch size for background jobs worker

Bug 32481: Limit prefetch size for background jobs worker

This patch adds a prefetch size of 1 to the background jobs worker,
so that it fetches 1 message at a time. Without this change,
the RabbitMQ connection timeout when too many messages for slow tasks
are fetched at the same time.

To test:
0. Apply patch
1. Run background worker
2. Rapidly enqueue multiple jobs that in total will take longer
than 30 minutes to process

Bug 32481: Use correct prefetch syntax for RabbitMQ

According to https://www.rabbitmq.com/stomp.html the header to
use for managing the prefetch is "prefetch-count".

You can verify the number of delivered and unacknowledged messages
on a channel on a connection by running "rabbitmqctl list_channels"
on the RabbitMQ host. This will tell you how many messages have been
delivered and are awaiting acknowledgement

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>