django slow log no longer offloads to celery

[nijotz]

django slow log was modified to log the record on request and update it on response. In doing so, it no longer offloads to celery, even if configured to do so. It would be nice to keep the "initial save on request, update on response" feature and to be able to offload both database transactions to celery.

It seems like the offload configuration should be updated to support specifying which database transaction to offload: the pre-request, the pre-response, both or none.

Race conditions between the two offloaded tasks would need to be avoided. The pre-response could be passed the celery task ID of the pre-request and make sure it has completed before attempting to update.