[Task] Ensure session data is saved before any backend process is invoked

[mradamcox]

Describe the task

The session "run" methods, which are passed to celery for backend processing, need to be independent of any session.data save operations: session.data should be saved before the task is handed off to celery. Additionally, a session status "queued" should be introduced so that cleanup tasks won't remove sessions that have been submitted to a broken celery deploy.

This is because celery task completion relies on rabbitmq so if the latter is misconfigured the task submissions could be lost.

(Some of this may already be in place, but this ticket is to check in and solidify them.)