Explicitly forbid mutating agg jobs, collections

Resolves #406

draft-ietf-ppm-dap.md

@@ -1461,6 +1461,12 @@ The Helper responds to the Leader with HTTP status code 201 Created and a body
 consisting of the `AggregationJobResp`, with media type
 "application/dap-aggregation-job-resp".
 
+Aggregation job initialization MUST be idempotent. That is, further requests to
+initialize some aggregation job whose body is identical to the first request
+MUST succeed. However, changing an aggregation job's parameters is illegal, so
+further requests with a different body MUST fail with an HTTP client error
+status code.
+
 Finally, if `state == Continued(prep_state)`, then the Helper stores `state` to
 prepare for the next continuation step ({{aggregation-helper-continuation}}).
 Otherwise, if `state == Finished(out_share)`, then the Helper stores `out_share`
@@ -1871,6 +1877,11 @@ The Leader then begins working with the Helper to aggregate the reports
 satisfying the query (or continues this process, depending on the VDAF) as
 described in {{aggregate-flow}}.
 
+Collection job creation MUST be idempotent. That is, further requests to
+create some collection job whose body is identical to the first request MUST
+succeed. However, changing a collection job's parameters is illegal, so further
+requests with a different body MUST fail with an HTTP client error status code.
+
 After receiving the response to its `CollectionReq`, the Collector makes an HTTP
 `POST` request to the collection job URI to check on the status of the collect
 job and eventually obtain the result. If the collection job is not finished