In AF-2635 we saw that the MongodSessionCatalog thread can crash during shutdown, this seems to be because we invariant that this status for the thread pool task is ok

Looking at the task definition one of the times that this status can be non-ok is during shutdown, so we are hitting this invariant and crashing the server during clean shutdown. This ticket should investigate if we really should be invarianting here (that killSessionTokens cannot occur during shutdown), in which case we need to prevent the task from being scheduled