fix logic behind get_session_key
fix logic behind get_session_key

- celery get_current_task() may have the same id between different tasks (!)
- hash resulting session "id" so that similar numbers end up very different, especially because we're truncating

dimagi/commcare-hq,dimagi/commcare-hq,dimagi/commcare-hq,dimagi/commcare-hq,dimagi/commcare-hq