Move lookback limit into the QueuedJobsIterator

[d80tb7]

Move the lookback limit up the stack from the QueuedGangIterator to the QueuedJobsIterator. This has a couple of benefits:

• Simplifies the QueuedGangIterator, which is already fairly complex
• Means we can get rid of all the logic about when "Evicted" jobs need to contribute to the lookback. QueuedJobsIterator only ever contains queued jobs (as opposed to evicted jobs) so the lookback always applies.