hydra-queue-runner: Allow concurrent notifications

The queue runner can now run up to ‘max-concurrent-notifications’ in parallel (default is 2). This is useful when some hydra-notify invocations can take a long time to complete (e.g. because they need to compress a giant build log) and we don't want this to block all other notifications.

Created by  Eelco Dolstra  on July 24, 2017
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