Queued tasks could deadlock in combination with secede

[gjoseph92]

When a task secedes, we aren't scheduling any queued tasks right now. So if a root-ish task calls secede, and all workers are full, the next task on the queue won't be scheduled, even though a slot has opened up on the worker. This could cause a deadlock if the long-running task has created another task and is waiting for it to run, but it's stuck in the queue.