Repeated requests `PUT /tasks/{tasks}/aggregation_jobs/{aggregation-job-id}` are handled as fresh aggregation jobs

[cjpatton]

DAP wants to allow the Leader to repeat requests to:

PUT /tasks/{tasks}/aggregation_jobs/{aggregation-job-id}

As long as the body of the request is the same, we should respond with the same AggregationJobRep as we did the first time the aggregation job was initialized.

At the moment, we don't keep track of past aggregation jobs: we would handle the repeated request as if it were fresh and respond by rejecting each of the reports.