Allow multiple workers for the same queue and configuration

[rosa]

From @djmb:

A single worker won't use more than 1 CPU core, so if you need to
dedicate more than on CPU core on a VM to a queue you'll need multiple
workers. I think I'll need this for the new campfire

This changes the configuration format to be like this:

workers:
    - queues: haystack_production_incineration
      processes: 3
      threads: 4
      polling_interval: 2
    - queues: haystack_production_recycling
      threads: 3
      polling_interval: 5

processes is 1 by default, and queues should be now an array of queues (which can include wildcards).