hydra-queue-runner: Start as many builds as possible on each iteration

Because we don't start a build if a dependency is already building, it's possible that some or all of the $extraAllowed highest-priority builds in the queue are not eligible. E.g. with $extraAllowed = 32, we might start only 3 builds even though there are thousands in the queue. The fix is to try all queued builds until $extraAllowed have been started.

Issue #99.

Created by  Eelco Dolstra  on June 7, 2013
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