We have some queues that are not only not latency-sensitive, they have
no particular urgency at all. This means that we need more than a
boolean field, so this changes:

1. `latency_sensitive_worker!` to `urgency :high`.
2. The rest of the workers to `urgency :default` (when `urgency` is not
   set).

It allows for workers to be marked as `urgency :none` in future.