Improve transform date range handling

When a collector is run it will generally POST a single record into
backdrop. This will mean that the earliest and latest values sent
through to the transform will be identical. When working with data that
is aggregated by period this will result in no data being returned, so
if this is the case we should change the request to pull in the latest
period using the timestamp as the end_at. This should result in use
pulling out the aggregated period including the current timestamp.