Fix stack overflow bug in the task system (#1583)

* Turn each task queue in the task system into a finite set of queues of increasing priority. The number of queues is a compile-time constant, currently two.
* When a task of priority higher than that of the highest priority queue is scheduled, execute it synchronously.
* When scheduling tasks in a task group, use a priority one higher than that of the calling task; when waiting on the task group, work only on tasks with this priority or higher. This is sufficient to avoid the deep recursion issue seen in issue #1570.

Fixes #1570.
Co-authored-by: Sam Yates <yates@cscs.ch>