Specify persistence of Activity resources

Each Activity resource MUST be persisted by a server (including over a restart) for a minimum of 24 hours after completion, or until explicitly deleted by a client Clients SHOULD delete each Activity created by a long-running operation soon after the client has seen the completion of the activity, or when the client is no longer interested in the result. Servers MAY deny clients access to delete Activity resources including but not limited to activities still in progress, or activities not created by that client, or MAY require clients to have a specific role or permission to delete activities.