Allow querying of waits in intervals

It would be helpful if the blocked-by graph (and the root-blocker graph proposed in #552) were able to query over intervals, in the same way as the `/traffic-signals/get-delays/` API call. It's hard to compare apples to apples, for instance, if one wanted to determine how long the car at the front of a queue was held up versus a car further back in the queue.