feat(identifiers): add function to list latest identifiers

Adds orangeqs.juice.identifiers.list_latest_ids and orangeqs.juice.identifiers.list_latest_run_ids, to give us a list of all identifiers in a window.

Verified on Borealis:

To prevent too much duplication of code, I've refactored the query part of _update_from_influxdb into a separate function that returns the list of all queried Identifier instances. This also adds limit as an argument that can be varied, and is set to 1 for _update_from_influxdb. It was the cleanest approach I could find, but I'd be happy to hear if there are better ways to do this!