[pipeline] Use priority queues

[graphicore]

rabbitmq supports this already.

• drag and drop jobs should always have priority, because a human will sit in front of the UI and wait. When the whole collection is being tested, it could take a while until the test starts.
• identified long-running families could be prioritized over normal families, so that the overall test run ends earlier. Some CJK families take a long time in FontValidator.

Combining both examples: DnD priority 2; CJk priority 1;everything else priority 0

We can use more priority levels of course.