BuildQueue should allow for multiple instances of same task

[kornysietsma]

The BuildQueue.get_details() method assumes there will only be one queued item for a given task, if there are multiple queued items it will return the last one.

This scenario is entirely possible - we have a single integration test job that is triggered by several upstream jobs, and if people commit to more than one upstream build we can easily see 2 or 3 queued jobs; in fact I'm trying to write code to detect this specific scenario!

(I'd raise a PR, but this requires some thoughts about how it would impact other methods that call get_details - it might be a breaking change for some users.)