Final profiles of queries should be reliably retrievable

During a code review, tarmstrong pointed out that there may be a problem with the query life cycle management which prevents the final profile from being accessible reliably for a query.

In particular, the ExecSummary and Error sections are not added to the profile until a query is unregistered. At which point, the profile is archived. It's possible that the archive log rolls over before the profile is retrieved or that the profile archive log has been disabled. In which case, there is no way to get the final profile of a query. Ideally, the final profile should be available before query unregistration (e.g. once all backends have completed).