Option to listen to multiple queues in parallel instead of in sequence

[ryanwitt]

Right now, qdone listens on one queue for --wait-time seconds, then moves on to the next queue.

We could get lower latency by listening to all queues, returning when we find data on any of them, and abort()ing the listen requests to the other queues.

This could potentially starve queues if one consistently wins the race for returning data first.

[ryanwitt]

To some degree, the latency problem can be minimized by using the new --active-only flag in 1.3.0, since it listens only to queues that had data available at the beginning of the listening round.