Fix queue_status RPC method

Seems I've missed one issue in the last pull request by Stefano  - RPC method for getting queue status is not working. It's because timestamp field in queue items cannot be serialized to json directly. Also the format of queue_status response is changed. This pull request fixes it.

Shortly:
- the get_status method of PriorityQueue now returns raw item (to be able to use it in callers).
- the queue_status method of TriggeredService does default conversion to json-encodable format
- the queue_status method of EvaluationService replaces the default conversion with the one that needed for AWS