make random acquire queues work less random

Queues feeding workers like rred are created in a random pattern to get
a few of them to run in parallel – but if we already have an idling queue
we don't need to assign it to a (potentially new) random queue as that
saves us the (agruably small) overhead of starting up a new queue,
avoids adding jobs to an already busy queue while others idle and as
a bonus reduces the size of debug logs a bit.

We also keep starting new queues now until we reach our limit before
we assign work at random to them, which should give us a more effective
utilisation overall compared to potentially adding work to busy queues
while we haven't reached our queue limit yet.

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