Get operation from active instead of complete spans

[quinnmil]

In the operation_utils.py, we first try to get the current operation from the operation set on the tracked_request. However, this requires an updated version of the scout-apm package.
For users who are on an older version of the agent, we fallback to attempting to get the operation via the "completed_spans" of the tracked_request. This is incorrect, as we should be instead iterating through the "active_spans".