Move scheduling key check into the QueuedJobsIterator

[d80tb7]

The scheduling key check used to be in theQueuedGangIterator. I've moved it to to the QueuedJobsIterator because:

• It simplifies QueuedGangIterator
• It means we exit asap if we know a job is unscheduable.
• The QueuedJobsIterator is only used for Queued (i.e. not evicted) jobs. This means we don't have to mess around with invalidating and skipping scheduling keys.